diff --git a/backend/migrations/20260126235947_lowercase_emails.down.sql b/backend/migrations/20260126235947_lowercase_emails.down.sql
new file mode 100644
index 0000000000..10766a8a0d
--- /dev/null
+++ b/backend/migrations/20260126235947_lowercase_emails.down.sql
@@ -0,0 +1,3 @@
+-- This migration cannot be reversed as the original email casing is not preserved
+-- Emails will remain lowercased
+SELECT 1;
diff --git a/backend/migrations/20260126235947_lowercase_emails.up.sql b/backend/migrations/20260126235947_lowercase_emails.up.sql
new file mode 100644
index 0000000000..e4d74dc1bc
--- /dev/null
+++ b/backend/migrations/20260126235947_lowercase_emails.up.sql
@@ -0,0 +1,14 @@
+-- Normalize emails to lowercase for consistency with external login normalization
+-- This migration lowercases email columns in critical authentication tables
+
+-- Lowercase emails in password table (primary user identity)
+UPDATE password SET email = LOWER(email) WHERE email != LOWER(email);
+
+-- Lowercase emails in usr table (workspace users)
+UPDATE usr SET email = LOWER(email) WHERE email != LOWER(email);
+
+-- Lowercase emails in email_to_igroup table (instance group memberships)
+UPDATE email_to_igroup SET email = LOWER(email) WHERE email != LOWER(email);
+
+-- Lowercase emails in token table (active sessions)
+UPDATE token SET email = LOWER(email) WHERE email != LOWER(email);
diff --git a/backend/windmill-common/src/oauth2.rs b/backend/windmill-common/src/oauth2.rs
index ed78514cdf..faa1f1b942 100644
--- a/backend/windmill-common/src/oauth2.rs
+++ b/backend/windmill-common/src/oauth2.rs
@@ -28,6 +28,24 @@ lazy_static::lazy_static! {
         .and_then(|x| x.parse::<bool>().ok())
         .unwrap_or(false));
 
+    /// Domain to append to emails missing a domain during external login (OAuth/SAML).
+    /// If set, emails without '@' will have '@{LOGIN_DOMAIN}' appended.
+    /// Example: LOGIN_DOMAIN=example.com transforms "john" to "john@example.com"
+    pub static ref LOGIN_DOMAIN: Option<String> = std::env::var("LOGIN_DOMAIN").ok();
+
+}
+
+/// Normalize an email from external login by appending LOGIN_DOMAIN if the email is missing a domain.
+/// Returns the email lowercased.
+pub fn normalize_external_email(email: &str) -> String {
+    let email = email.trim();
+    if email.contains('@') {
+        email.to_lowercase()
+    } else if let Some(domain) = LOGIN_DOMAIN.as_ref() {
+        format!("{}@{}", email, domain).to_lowercase()
+    } else {
+        email.to_lowercase()
+    }
 }
 
 #[derive(Serialize)]
@@ -41,3 +59,41 @@ pub enum InstanceEvent {
     UserInvitedWorkspace { workspace: String, email: String },
     UserJoinedWorkspace { workspace: String, email: String, username: String },
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_normalize_external_email_with_domain() {
+        // Email already has domain - should just lowercase
+        assert_eq!(
+            normalize_external_email("John.Doe@Example.COM"),
+            "john.doe@example.com"
+        );
+        assert_eq!(
+            normalize_external_email("user@domain.org"),
+            "user@domain.org"
+        );
+    }
+
+    #[test]
+    fn test_normalize_external_email_trims_whitespace() {
+        assert_eq!(
+            normalize_external_email("  user@example.com  "),
+            "user@example.com"
+        );
+        assert_eq!(normalize_external_email("  john  "), "john");
+    }
+
+    #[test]
+    fn test_normalize_external_email_without_domain_no_env() {
+        // When LOGIN_DOMAIN is not set, email without @ stays as-is (lowercased)
+        // Note: This test's behavior depends on whether LOGIN_DOMAIN env var is set
+        // In the test environment, it's typically not set
+        let result = normalize_external_email("JohnDoe");
+        // Result will either be "johndoe" or "johndoe@{LOGIN_DOMAIN}" depending on env
+        assert!(result.starts_with("johndoe"));
+        assert_eq!(result, result.to_lowercase());
+    }
+}