diff --git a/pageserver/src/pgdatadir_mapping.rs b/pageserver/src/pgdatadir_mapping.rs
index 336d1c3fb8a5..25d00d6dfd0d 100644
--- a/pageserver/src/pgdatadir_mapping.rs
+++ b/pageserver/src/pgdatadir_mapping.rs
@@ -919,6 +919,14 @@ impl Timeline {
             result.add_key(AUX_FILES_KEY);
         }
 
+        #[cfg(test)]
+        {
+            let guard = self.extra_test_dense_keyspace.load();
+            for kr in &guard.ranges {
+                result.add_range(kr.clone());
+            }
+        }
+
         Ok((
             result.to_keyspace(),
             /* AUX sparse key space */
diff --git a/pageserver/src/tenant.rs b/pageserver/src/tenant.rs
index f9ed6d3071eb..d556f7233545 100644
--- a/pageserver/src/tenant.rs
+++ b/pageserver/src/tenant.rs
@@ -5264,6 +5264,9 @@ mod tests {
         let cancel = CancellationToken::new();
 
         let mut test_key = Key::from_hex("010000000033333333444444445500000000").unwrap();
+        let mut test_key_end = test_key;
+        test_key_end.field6 = NUM_KEYS as u32;
+        tline.add_extra_test_dense_keyspace(KeySpace::single(test_key..test_key_end));
 
         let mut keyspace = KeySpaceAccum::new();
 
@@ -6223,8 +6226,8 @@ mod tests {
 
         let cancel = CancellationToken::new();
 
-        let mut base_key = Key::from_hex("000000000033333333444444445500000000").unwrap();
-        base_key.field1 = AUX_KEY_PREFIX;
+        let base_key = Key::from_hex("620000000033333333444444445500000000").unwrap();
+        assert_eq!(base_key.field1, AUX_KEY_PREFIX); // in case someone accidentally changed the prefix...
         let mut test_key = base_key;
         let mut lsn = Lsn(0x10);
 
@@ -6329,6 +6332,7 @@ mod tests {
                 Lsn(0x20), // it's fine to not advance LSN to 0x30 while using 0x30 to get below because `get_vectored_impl` does not wait for LSN
             )
             .await?;
+        tline.add_extra_test_dense_keyspace(KeySpace::single(base_key..(base_key_nonexist.next())));
 
         let child = tenant
             .branch_timeline_test_with_layers(
diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index 54a4ceeaf3a3..28627e79117c 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -426,6 +426,14 @@ pub struct Timeline {
 
     /// Indicate whether aux file v2 storage is enabled.
     pub(crate) last_aux_file_policy: AtomicAuxFilePolicy,
+
+    /// Some test cases directly place keys into the timeline without actually modifying the directory
+    /// keys (i.e., DB_DIR). The test cases creating such keys will put the keyspaces here, so that
+    /// these keys won't get garbage-collected during compaction/GC. This field only modifies the dense
+    /// keyspace return value of `collect_keyspace`. For sparse keyspaces, use AUX keys for testing, and
+    /// in the future, add `extra_test_sparse_keyspace` if necessary.
+    #[cfg(test)]
+    pub(crate) extra_test_dense_keyspace: ArcSwap<KeySpace>,
 }
 
 pub struct WalReceiverInfo {
@@ -2344,6 +2352,9 @@ impl Timeline {
                 aux_file_size_estimator: AuxFileSizeEstimator::new(aux_file_metrics),
 
                 last_aux_file_policy: AtomicAuxFilePolicy::new(aux_file_policy),
+
+                #[cfg(test)]
+                extra_test_dense_keyspace: ArcSwap::new(Arc::new(KeySpace::default())),
             };
             result.repartition_threshold =
                 result.get_checkpoint_distance() / REPARTITION_FREQ_IN_CHECKPOINT_DISTANCE;
@@ -5562,6 +5573,13 @@ impl Timeline {
         }
         Ok(layers)
     }
+
+    #[cfg(test)]
+    pub(crate) fn add_extra_test_dense_keyspace(&self, ks: KeySpace) {
+        let mut keyspace = self.extra_test_dense_keyspace.load().as_ref().clone();
+        keyspace.merge(&ks);
+        self.extra_test_dense_keyspace.store(Arc::new(keyspace));
+    }
 }
 
 type TraversalPathItem = (ValueReconstructResult, Lsn, TraversalId);