diff --git a/pageserver/src/tenant/timeline/detach_ancestor.rs b/pageserver/src/tenant/timeline/detach_ancestor.rs
index 641faada25..b4c0ab0329 100644
--- a/pageserver/src/tenant/timeline/detach_ancestor.rs
+++ b/pageserver/src/tenant/timeline/detach_ancestor.rs
@@ -29,6 +29,9 @@ pub(crate) enum Error {
     #[error("shutting down, please retry later")]
     ShuttingDown,
 
+    #[error("archived: {}", .0)]
+    Archived(TimelineId),
+
     #[error(transparent)]
     NotFound(crate::tenant::GetTimelineError),
 
@@ -79,8 +82,9 @@ impl From<Error> for ApiError {
     fn from(value: Error) -> Self {
         match value {
             Error::NoAncestor => ApiError::Conflict(value.to_string()),
-            Error::TooManyAncestors => ApiError::BadRequest(anyhow::anyhow!("{}", value)),
+            Error::TooManyAncestors => ApiError::BadRequest(anyhow::anyhow!("{value}")),
             Error::ShuttingDown => ApiError::ShuttingDown,
+            Error::Archived(_) => ApiError::BadRequest(anyhow::anyhow!("{value}")),
             Error::OtherTimelineDetachOngoing(_) | Error::FailedToReparentAll => {
                 ApiError::ResourceUnavailable(value.to_string().into())
             }
@@ -201,12 +205,18 @@ pub(super) async fn prepare(
         }));
     };
 
+    if detached.is_archived() != Some(false) {
+        return Err(Archived(detached.timeline_id));
+    }
+
     if !ancestor_lsn.is_valid() {
         // rare case, probably wouldn't even load
         tracing::error!("ancestor is set, but ancestor_lsn is invalid, this timeline needs fixing");
         return Err(NoAncestor);
     }
 
+    check_no_archived_children_of_ancestor(tenant, detached, &ancestor, ancestor_lsn)?;
+
     if ancestor.ancestor_timeline.is_some() {
         // non-technical requirement; we could flatten N ancestors just as easily but we chose
         // not to, at least initially
@@ -950,3 +960,36 @@ where
         }
     })
 }
+
+fn check_no_archived_children_of_ancestor(
+    tenant: &Tenant,
+    detached: &Arc<Timeline>,
+    ancestor: &Arc<Timeline>,
+    ancestor_lsn: Lsn,
+) -> Result<(), Error> {
+    let timelines = tenant.timelines.lock().unwrap();
+    let timelines_offloaded = tenant.timelines_offloaded.lock().unwrap();
+    for timeline in reparentable_timelines(timelines.values(), detached, ancestor, ancestor_lsn) {
+        if timeline.is_archived() == Some(true) {
+            return Err(Error::Archived(timeline.timeline_id));
+        }
+    }
+    for timeline_offloaded in timelines_offloaded.values() {
+        if timeline_offloaded.ancestor_timeline_id != Some(ancestor.timeline_id) {
+            continue;
+        }
+        // This forbids the detach ancestor feature if flattened timelines are present,
+        // even if the ancestor_lsn is from after the branchpoint of the detached timeline.
+        // But as per current design, we don't record the ancestor_lsn of flattened timelines.
+        // This is a bit unfortunate, but as of writing this we don't support flattening
+        // anyway. Maybe we can evolve the data model in the future.
+        if let Some(retain_lsn) = timeline_offloaded.ancestor_retain_lsn {
+            let is_earlier = retain_lsn <= ancestor_lsn;
+            if !is_earlier {
+                continue;
+            }
+        }
+        return Err(Error::Archived(timeline_offloaded.timeline_id));
+    }
+    Ok(())
+}
diff --git a/test_runner/regress/test_timeline_detach_ancestor.py b/test_runner/regress/test_timeline_detach_ancestor.py
index 0c8554bb54..d467c59e62 100644
--- a/test_runner/regress/test_timeline_detach_ancestor.py
+++ b/test_runner/regress/test_timeline_detach_ancestor.py
@@ -9,7 +9,7 @@ from queue import Empty, Queue
 from threading import Barrier
 
 import pytest
-from fixtures.common_types import Lsn, TimelineId
+from fixtures.common_types import Lsn, TimelineArchivalState, TimelineId
 from fixtures.log_helper import log
 from fixtures.neon_fixtures import (
     LogCursor,
@@ -634,7 +634,13 @@ def test_timeline_ancestor_detach_errors(neon_env_builder: NeonEnvBuilder, shard
     shards = 2 if sharded else 1
 
     neon_env_builder.num_pageservers = shards
-    env = neon_env_builder.init_start(initial_tenant_shard_count=shards if sharded else None)
+    env = neon_env_builder.init_start(
+        initial_tenant_shard_count=shards if sharded else None,
+        initial_tenant_conf={
+            # turn off gc, we want to do manual offloading here.
+            "gc_period": "0s",
+        },
+    )
 
     pageservers = dict((int(p.id), p) for p in env.pageservers)
 
@@ -656,7 +662,9 @@ def test_timeline_ancestor_detach_errors(neon_env_builder: NeonEnvBuilder, shard
         client.detach_ancestor(env.initial_tenant, env.initial_timeline)
     assert info.value.status_code == 409
 
-    _ = env.create_branch("first_branch")
+    early_branch = env.create_branch("early_branch")
+
+    first_branch = env.create_branch("first_branch")
 
     second_branch = env.create_branch("second_branch", ancestor_branch_name="first_branch")
 
@@ -665,6 +673,29 @@ def test_timeline_ancestor_detach_errors(neon_env_builder: NeonEnvBuilder, shard
         client.detach_ancestor(env.initial_tenant, second_branch)
     assert info.value.status_code == 400
 
+    client.timeline_archival_config(
+        env.initial_tenant, second_branch, TimelineArchivalState.ARCHIVED
+    )
+
+    client.timeline_archival_config(
+        env.initial_tenant, early_branch, TimelineArchivalState.ARCHIVED
+    )
+
+    with pytest.raises(PageserverApiException, match=f".*archived: {early_branch}") as info:
+        client.detach_ancestor(env.initial_tenant, first_branch)
+    assert info.value.status_code == 400
+
+    if not sharded:
+        client.timeline_offload(env.initial_tenant, early_branch)
+
+    client.timeline_archival_config(
+        env.initial_tenant, first_branch, TimelineArchivalState.ARCHIVED
+    )
+
+    with pytest.raises(PageserverApiException, match=f".*archived: {first_branch}") as info:
+        client.detach_ancestor(env.initial_tenant, first_branch)
+    assert info.value.status_code == 400
+
 
 def test_sharded_timeline_detach_ancestor(neon_env_builder: NeonEnvBuilder):
     """