diff --git a/src/main/browser/browser-cookie-chromium-scan.ts b/src/main/browser/browser-cookie-chromium-scan.ts
index e3bdc6a6409..eb057712a11 100644
--- a/src/main/browser/browser-cookie-chromium-scan.ts
+++ b/src/main/browser/browser-cookie-chromium-scan.ts
@@ -7,7 +7,7 @@ import {
 } from './browser-cookie-import-policy'
 import { prepareStagedCookiesForImport } from './browser-cookie-staged-import'
 import { chromiumTimestampToUnix, buildChromiumCookieInsertParams } from './browser-cookie-sqlite'
-import { chromiumSameSite } from './browser-cookie-validation'
+import { databaseSameSite } from './browser-cookie-validation'
 import {
   buildUndecryptableWarning,
   cookieEncryptionVersion,
@@ -97,7 +97,8 @@ export function scanChromiumCookieRows(
     const path = sourceRow.path as string
     const secure = sourceRow.is_secure === 1n
     const httpOnly = sourceRow.is_httponly === 1n
-    const sameSite = chromiumSameSite(Number(sourceRow.samesite ?? 0))
+    // Why: pre-samesite schemas and NULL rows follow Chromium's own unspecified fallback.
+    const sameSite = databaseSameSite(Number(sourceRow.samesite ?? -1))
     const expiresUtc = chromiumTimestampToUnix(sourceRow.expires_utc as bigint)
     const partition = partitionBySourceRow.get(sourceRow)!
     // Why: cookie values are raw bytes, not UTF-8; latin1 preserves 0x00–0xFF without lossy replacement.
diff --git a/src/main/browser/browser-cookie-firefox-import.ts b/src/main/browser/browser-cookie-firefox-import.ts
index 60482b30e31..d7ada7e76de 100644
--- a/src/main/browser/browser-cookie-firefox-import.ts
+++ b/src/main/browser/browser-cookie-firefox-import.ts
@@ -9,7 +9,7 @@ import {
   cookieImportTarget,
   type CookieImportOptions
 } from './browser-cookie-import-pipeline'
-import { deriveUrl, firefoxSameSite, type ValidatedCookie } from './browser-cookie-validation'
+import { databaseSameSite, deriveUrl, type ValidatedCookie } from './browser-cookie-validation'
 import type { DetectedBrowser } from './browser-cookie-detection-types'
 import { diag } from './browser-cookie-import-diagnostics'
 
@@ -108,7 +108,7 @@ export async function importCookiesFromFirefox(
         path: row.path || '/',
         secure,
         httpOnly: row.isHttpOnly === 1,
-        sameSite: firefoxSameSite(row.sameSite),
+        sameSite: databaseSameSite(row.sameSite),
         expirationDate: row.expiry > 0 ? row.expiry : undefined,
         partition: readFirefoxRowPartition(row, firefoxColumns)
       })
diff --git a/src/main/browser/browser-cookie-import-test-database.ts b/src/main/browser/browser-cookie-import-test-database.ts
index 31cdfc1f157..d3617928fb2 100644
--- a/src/main/browser/browser-cookie-import-test-database.ts
+++ b/src/main/browser/browser-cookie-import-test-database.ts
@@ -13,7 +13,7 @@ type ChromiumCookieTestRow = {
   hasCrossSiteAncestor?: 0 | 1
   isSecure?: 0 | 1
   isHttpOnly?: 0 | 1
-  sameSite?: 0 | 1 | 2 | 3
+  sameSite?: -1 | 0 | 1 | 2 | 3 | null
 }
 
 export function createChromiumCookieTestDatabase(
@@ -38,7 +38,7 @@ export function createChromiumCookieTestDatabase(
       expires_utc INTEGER NOT NULL,
       is_secure INTEGER NOT NULL,
       is_httponly INTEGER NOT NULL,
-      samesite INTEGER NOT NULL,
+      samesite INTEGER,
       source_scheme INTEGER NOT NULL DEFAULT 0,
       source_port INTEGER NOT NULL DEFAULT -1,
       last_update_utc INTEGER NOT NULL DEFAULT 0,
@@ -75,7 +75,7 @@ export function createChromiumCookieTestDatabase(
       row.encryptedValue ?? Buffer.alloc(0),
       row.isSecure ?? 0,
       row.isHttpOnly ?? 0,
-      row.sameSite ?? 0,
+      row.sameSite === undefined ? -1 : row.sameSite,
       0,
       row.hasCrossSiteAncestor ?? 0
     )
diff --git a/src/main/browser/browser-cookie-samesite.electron.test.ts b/src/main/browser/browser-cookie-samesite.electron.test.ts
new file mode 100644
index 00000000000..4f04f93c72e
--- /dev/null
+++ b/src/main/browser/browser-cookie-samesite.electron.test.ts
@@ -0,0 +1,268 @@
+import { spawnSync } from 'node:child_process'
+import { existsSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
+import { createRequire } from 'node:module'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { DatabaseSync } from 'node:sqlite'
+import { afterAll, beforeAll, describe, expect, it } from 'vitest'
+import { build as buildVite } from 'vite'
+import { createChromiumCookieTestDatabase } from './browser-cookie-import-test-database'
+
+type CookieSameSite = 'unspecified' | 'no_restriction' | 'lax' | 'strict'
+
+type ExpectedCookie = {
+  name: string
+  rawSameSite: -1 | 0 | 1 | 2
+  secure: boolean
+  sameSite: CookieSameSite
+}
+
+type JarCookie = Pick<ExpectedCookie, 'name' | 'secure' | 'sameSite'>
+
+type ImportResult = {
+  ok: boolean
+  reason?: string
+  summary?: { importedCookies: number; skippedCookies: number }
+}
+
+type FixtureResult = {
+  step: string
+  error?: string
+  beforeCookieCount: number
+  importResult: ImportResult
+  afterCookies: JarCookie[]
+}
+
+type SourceShape = {
+  name: string
+  samesite: number | null
+  is_secure: number
+}
+
+const electronBinary = createRequire(import.meta.url)('electron') as string
+const fixtureRoots: string[] = []
+
+const VALID_COMBINATIONS: readonly ExpectedCookie[] = [
+  {
+    name: 'raw-minus-1-secure-0',
+    rawSameSite: -1,
+    secure: false,
+    sameSite: 'unspecified'
+  },
+  // Ablation C: neither the old decoder nor the null-default regression affects this row.
+  {
+    name: 'raw-minus-1-secure-1',
+    rawSameSite: -1,
+    secure: true,
+    sameSite: 'unspecified'
+  },
+  { name: 'raw-0-secure-1', rawSameSite: 0, secure: true, sameSite: 'no_restriction' },
+  { name: 'raw-1-secure-0', rawSameSite: 1, secure: false, sameSite: 'lax' },
+  { name: 'raw-1-secure-1', rawSameSite: 1, secure: true, sameSite: 'lax' },
+  { name: 'raw-2-secure-0', rawSameSite: 2, secure: false, sameSite: 'strict' },
+  { name: 'raw-2-secure-1', rawSameSite: 2, secure: true, sameSite: 'strict' }
+]
+
+const REJECTION_CONTROL = {
+  name: 'raw-0-secure-0',
+  rawSameSite: 0,
+  secure: false
+} as const
+
+const NULL_CASE = {
+  name: 'raw-null-secure-0',
+  rawSameSite: null,
+  secure: false,
+  sameSite: 'unspecified'
+} as const
+
+afterAll(() => {
+  for (const root of fixtureRoots) {
+    rmSync(root, { recursive: true, force: true, maxRetries: 5, retryDelay: 100 })
+  }
+})
+
+function buildFixtureMain(bundlePath: string, resultPath: string, sourceDbPath: string): string {
+  return `
+const { app, BrowserWindow, session } = require('electron')
+const { writeFileSync } = require('node:fs')
+const { importCookiesFromBrowser } = require(${JSON.stringify(bundlePath)})
+const resultPath = ${JSON.stringify(resultPath)}
+let currentStep = 'starting'
+
+const mark = (step) => {
+  currentStep = step
+  writeFileSync(resultPath, JSON.stringify({ step }))
+}
+
+async function run() {
+  const timeout = setTimeout(() => {
+    writeFileSync(resultPath, JSON.stringify({ step: 'timed out after ' + currentStep }))
+    app.exit(1)
+  }, 30000)
+  await app.whenReady()
+  mark('ready')
+  const partition = 'persist:samesite-enum-cookie-test'
+  const targetSession = session.fromPartition(partition)
+  const window = new BrowserWindow({ show: false, webPreferences: { partition } })
+  mark('window created')
+  await window.loadURL('data:text/html,<title>same-site enum fixture</title>')
+  mark('window loaded')
+  const beforeCookieCount = (await targetSession.cookies.get({})).length
+
+  const importResult = await importCookiesFromBrowser(
+    {
+      family: 'chrome',
+      label: 'Google Chrome',
+      cookiesPath: ${JSON.stringify(sourceDbPath)},
+      profiles: [],
+      selectedProfile: ''
+    },
+    partition
+  )
+  mark('import finished')
+
+  const afterCookies = (await targetSession.cookies.get({}))
+    .filter((cookie) => cookie.name.startsWith('raw-'))
+    .map((cookie) => ({
+      name: cookie.name,
+      sameSite: cookie.sameSite,
+      secure: cookie.secure
+    }))
+  clearTimeout(timeout)
+  writeFileSync(resultPath, JSON.stringify({
+    step: currentStep,
+    beforeCookieCount,
+    importResult,
+    afterCookies
+  }))
+  window.destroy()
+  app.exit(0)
+}
+
+run().catch((error) => {
+  writeFileSync(resultPath, JSON.stringify({ step: currentStep, error: String(error?.stack || error) }))
+  app.exit(1)
+})
+`
+}
+
+function readSourceShape(sourceDbPath: string): SourceShape[] {
+  const db = new DatabaseSync(sourceDbPath, { readOnly: true })
+  try {
+    return db
+      .prepare('SELECT name, samesite, is_secure FROM cookies ORDER BY rowid')
+      .all() as SourceShape[]
+  } finally {
+    db.close()
+  }
+}
+
+async function runFixture(): Promise<{ fixture: FixtureResult; sourceShape: SourceShape[] }> {
+  const root = mkdtempSync(join(tmpdir(), 'orca-samesite-enum-'))
+  fixtureRoots.push(root)
+  const bundlePath = join(root, 'cookie-import-samesite.cjs')
+  const bundleEntryPath = join(root, 'cookie-import-samesite.ts')
+  const resultPath = join(root, 'result.json')
+  const fixturePath = join(root, 'main.cjs')
+  const sourceDbPath = join(root, 'source-cookies.db')
+  const rows = [REJECTION_CONTROL, ...VALID_COMBINATIONS, NULL_CASE].map(
+    ({ name, rawSameSite, secure }) => ({
+      domain: '.samesite.example',
+      name,
+      value: 'synthetic-value',
+      isSecure: secure ? (1 as const) : (0 as const),
+      sameSite: rawSameSite
+    })
+  )
+  createChromiumCookieTestDatabase(sourceDbPath, rows).close()
+  const sourceShape = readSourceShape(sourceDbPath)
+  writeFileSync(
+    bundleEntryPath,
+    `export { importCookiesFromBrowser } from ${JSON.stringify(join(process.cwd(), 'src/main/browser/browser-cookie-import.ts'))}`
+  )
+  await buildVite({
+    configFile: false,
+    logLevel: 'silent',
+    build: {
+      emptyOutDir: false,
+      lib: {
+        entry: bundleEntryPath,
+        formats: ['cjs'],
+        fileName: () => 'cookie-import-samesite.cjs'
+      },
+      outDir: root,
+      target: 'node20',
+      rollupOptions: { external: ['electron', /^node:/] }
+    }
+  })
+  writeFileSync(fixturePath, buildFixtureMain(bundlePath, resultPath, sourceDbPath))
+  const { ELECTRON_RUN_AS_NODE: _electronRunAsNode, ...env } = process.env
+  const electronArgs = [fixturePath, `--user-data-dir=${join(root, 'profile')}`]
+  const executable = process.platform === 'linux' ? 'xvfb-run' : electronBinary
+  const args =
+    process.platform === 'linux'
+      ? ['--auto-servernum', electronBinary, ...electronArgs, '--no-sandbox']
+      : electronArgs
+  const run = spawnSync(executable, args, {
+    encoding: 'utf8',
+    env: { ...env, ORCA_BACKGROUND_LAUNCH: '1' },
+    timeout: 90_000
+  })
+  const fixtureResult = existsSync(resultPath) ? readFileSync(resultPath, 'utf8') : 'no result'
+  expect(run.error).toBeUndefined()
+  expect(run.status, `${fixtureResult}\n${run.stdout}\n${run.stderr}`).toBe(0)
+  return { fixture: JSON.parse(fixtureResult) as FixtureResult, sourceShape }
+}
+
+describe('Chromium SameSite storage enum import', () => {
+  let fixture: FixtureResult
+  let sourceShape: SourceShape[]
+
+  beforeAll(async () => {
+    ;({ fixture, sourceShape } = await runFixture())
+  }, 120_000)
+
+  it('runs the real Chromium import against the complete synthetic matrix', () => {
+    expect(fixture.step).toBe('import finished')
+    expect(fixture.beforeCookieCount).toBe(0)
+    expect(fixture.importResult.ok).toBe(true)
+    expect(sourceShape).toEqual(
+      [REJECTION_CONTROL, ...VALID_COMBINATIONS, NULL_CASE].map(
+        ({ name, rawSameSite, secure }) => ({
+          name,
+          samesite: rawSameSite,
+          is_secure: secure ? 1 : 0
+        })
+      )
+    )
+  })
+
+  it.each(VALID_COMBINATIONS)(
+    'imports $name with the decoded SameSite and authored Secure flag',
+    ({ name, sameSite, secure }) => {
+      expect(fixture.afterCookies.find((cookie) => cookie.name === name)).toEqual({
+        name,
+        sameSite,
+        secure
+      })
+    }
+  )
+
+  it('rejects the synthetic SameSite=None insecure control and continues later writes', () => {
+    // Chromium refuses this shape, so real profiles cannot contain it. Keeping the synthetic row
+    // proves the fixture can observe rejection instead of making every presence assertion vacuous.
+    expect(
+      fixture.afterCookies.find((cookie) => cookie.name === REJECTION_CONTROL.name)
+    ).toBeUndefined()
+    expect(fixture.afterCookies.find((cookie) => cookie.name === 'raw-2-secure-1')).toBeDefined()
+  })
+
+  it('imports a null SameSite column as unspecified without changing Secure', () => {
+    expect(fixture.afterCookies.find((cookie) => cookie.name === NULL_CASE.name)).toEqual({
+      name: NULL_CASE.name,
+      sameSite: NULL_CASE.sameSite,
+      secure: NULL_CASE.secure
+    })
+  })
+})
diff --git a/src/main/browser/browser-cookie-validation.test.ts b/src/main/browser/browser-cookie-validation.test.ts
new file mode 100644
index 00000000000..d7b065beeda
--- /dev/null
+++ b/src/main/browser/browser-cookie-validation.test.ts
@@ -0,0 +1,30 @@
+import { describe, expect, it } from 'vitest'
+import { databaseSameSite } from './browser-cookie-validation'
+
+describe('databaseSameSite', () => {
+  it.each([
+    { raw: -1, expected: 'unspecified' },
+    { raw: 0, expected: 'no_restriction' },
+    { raw: 1, expected: 'lax' },
+    { raw: 2, expected: 'strict' },
+    { raw: 3, expected: 'unspecified' },
+    // Why: 256 is Firefox's nsICookie SAMESITE_UNSET, written for every cookie with no SameSite
+    // attribute -- the most common shape in a modern Firefox profile. It reaches the default arm,
+    // so without this case the decoder's busiest Firefox input would be untested.
+    { raw: 256, expected: 'unspecified' },
+    { raw: 99, expected: 'unspecified' },
+    { raw: 1.5, expected: 'unspecified' }
+  ] as const)('decodes $raw as $expected', ({ raw, expected }) => {
+    expect(databaseSameSite(raw)).toBe(expected)
+  })
+
+  // Why: pre-v10 Firefox rows carry NULL, and the Chromium scan feeds `?? -1`. Both arrive here as
+  // a non-integer rather than a number, and both must be unspecified rather than None (0).
+  it.each([
+    { label: 'null', raw: null },
+    { label: 'undefined', raw: undefined },
+    { label: 'NaN', raw: Number.NaN }
+  ])('decodes $label as unspecified', ({ raw }) => {
+    expect(databaseSameSite(raw as unknown as number)).toBe('unspecified')
+  })
+})
diff --git a/src/main/browser/browser-cookie-validation.ts b/src/main/browser/browser-cookie-validation.ts
index 6ae543245ef..7f08f713f2e 100644
--- a/src/main/browser/browser-cookie-validation.ts
+++ b/src/main/browser/browser-cookie-validation.ts
@@ -25,21 +25,16 @@ export type ValidatedCookie = ImportedCookieFields & {
   partition: SourcePartitionRead
 }
 
-// Why: Chromium's CookieSameSiteForStorage enum (0=Unspecified,1=None,2=Lax,3=Strict) differs from Firefox's numbering.
-export function chromiumSameSite(raw: number): 'unspecified' | 'no_restriction' | 'lax' | 'strict' {
-  switch (raw) {
-    case 1:
-      return 'no_restriction'
-    case 2:
-      return 'lax'
-    case 3:
-      return 'strict'
-    default:
-      return 'unspecified'
-  }
-}
-
-export function firefoxSameSite(raw: number): 'unspecified' | 'no_restriction' | 'lax' | 'strict' {
+// Chromium stores net::CookieSameSite unchanged; see net/cookies/cookie_constants.h and
+// net/extras/sqlite/sqlite_persistent_cookie_store.cc (-1 unspecified, 0 None, 1 Lax, 2 Strict;
+// 3 is the deprecated EXTENDED value Chromium itself folds to unspecified).
+// Firefox's moz_cookies OVERLAPS on 1=Lax and 2=Strict but its domain is wider, so the default arm
+// is load-bearing for it, not incidental: 256 (nsICookie SAMESITE_UNSET) is what modern Firefox
+// writes for every cookie with no SameSite attribute, NULL appears on pre-v10 rows, and 0 means
+// explicit None OR a legacy unset row the schema-15 migration left behind — the two are not
+// distinguishable in the column. Every one of those must land on unspecified, so do NOT make this
+// switch exhaustive or drop the default without re-checking both browsers' real value domains.
+export function databaseSameSite(raw: number): 'unspecified' | 'no_restriction' | 'lax' | 'strict' {
   switch (raw) {
     case 0:
       return 'no_restriction'
@@ -56,7 +51,7 @@ export function normalizeSameSite(
   raw: unknown
 ): 'unspecified' | 'no_restriction' | 'lax' | 'strict' {
   if (typeof raw === 'number') {
-    return chromiumSameSite(raw)
+    return databaseSameSite(raw)
   }
   if (typeof raw !== 'string') {
     return 'unspecified'