diff --git a/services/identity/src/database.rs b/services/identity/src/database.rs
--- a/services/identity/src/database.rs
+++ b/services/identity/src/database.rs
@@ -20,6 +20,7 @@
 pub use crate::database::device_list::DeviceIDAttribute;
 pub use crate::database::one_time_keys::OTKRow;
 use crate::{
+  constants::RESERVED_USERNAMES_TABLE_USER_ID_INDEX,
   ddb_utils::EthereumIdentity, device_list::SignedDeviceList,
   grpc_services::shared::PlatformMetadata, log::redact_sensitive_data,
   reserved_users::UserDetail, siwe::SocialProof,
@@ -1229,20 +1230,70 @@
   ) -> Result<Option<String>, Error> {
     let username_lower = username.to_lowercase();
 
+    self
+      .query_reserved_usernames_table_index(
+        &username_lower,
+        (
+          RESERVED_USERNAMES_TABLE_USERNAME_LOWER_INDEX,
+          RESERVED_USERNAMES_TABLE_USERNAME_LOWER_ATTRIBUTE,
+        ),
+        attribute,
+      )
+      .await
+  }
+
+  pub async fn query_reserved_usernames_by_user_ids(
+    &self,
+    user_ids: Vec<String>,
+  ) -> Result<HashMap<String, String>, Error> {
+    if user_ids.len() > 50 {
+      tracing::warn!(
+        num_queries = user_ids.len(),
+        "Querying more than 50 reserved usernames by user ID! {}",
+        "This is inefficient and might lead to performance issues."
+      );
+    }
+    let mut results = HashMap::with_capacity(user_ids.len());
+    for user_id in user_ids {
+      let query_result = self
+        .query_reserved_usernames_table_index(
+          &user_id,
+          (
+            RESERVED_USERNAMES_TABLE_USER_ID_INDEX,
+            RESERVED_USERNAMES_TABLE_USER_ID_ATTRIBUTE,
+          ),
+          RESERVED_USERNAMES_TABLE_PARTITION_KEY,
+        )
+        .await?;
+
+      if let Some(username) = query_result {
+        results.insert(user_id, username);
+      }
+    }
+
+    Ok(results)
+  }
+
+  async fn query_reserved_usernames_table_index(
+    &self,
+    key_value: impl Into<String>,
+    // tuple of (index name, key attribute)
+    index_and_key: (&'static str, &'static str),
+    attribute: &str,
+  ) -> Result<Option<String>, Error> {
+    let (index, key_attr) = index_and_key;
     let response = self
       .client
       .query()
       .table_name(RESERVED_USERNAMES_TABLE)
-      .index_name(RESERVED_USERNAMES_TABLE_USERNAME_LOWER_INDEX)
-      .key_condition_expression("#username_lower = :username_lower")
-      .expression_attribute_names(
-        "#username_lower",
-        RESERVED_USERNAMES_TABLE_USERNAME_LOWER_ATTRIBUTE,
-      )
+      .index_name(index)
+      .key_condition_expression("#key_name = :key_value")
+      .expression_attribute_names("#key_name", key_attr)
       .expression_attribute_values(
-        ":username_lower",
-        AttributeValue::S(username_lower),
+        ":key_value",
+        AttributeValue::S(key_value.into()),
       )
+      .limit(1)
       .send()
       .await
       .map_err(|e| Error::AwsSdk(e.into()))?;
diff --git a/services/identity/src/grpc_services/authenticated.rs b/services/identity/src/grpc_services/authenticated.rs
--- a/services/identity/src/grpc_services/authenticated.rs
+++ b/services/identity/src/grpc_services/authenticated.rs
@@ -864,20 +864,32 @@
   ) -> Result<Response<UserIdentitiesResponse>, tonic::Status> {
     let message = request.into_inner();
 
-    let results = self
+    let users_table_results = self
       .db_client
-      .find_db_user_identities(message.user_ids)
+      .find_db_user_identities(message.user_ids.clone())
       .await
       .map_err(handle_db_error)?;
 
-    let mapped_results = results
+    // Look up only user IDs that haven't been found in users table
+    let reserved_user_ids_to_query: Vec<String> = message
+      .user_ids
+      .into_iter()
+      .filter(|user_id| !users_table_results.contains_key(user_id))
+      .collect();
+    let reserved_user_identifiers = self
+      .db_client
+      .query_reserved_usernames_by_user_ids(reserved_user_ids_to_query)
+      .await
+      .map_err(handle_db_error)?;
+
+    let identities = users_table_results
       .into_iter()
       .map(|(user_id, identifier)| (user_id, identifier.into()))
       .collect();
 
     let response = UserIdentitiesResponse {
-      identities: mapped_results,
-      reserved_user_identifiers: HashMap::new(),
+      identities,
+      reserved_user_identifiers,
     };
     return Ok(Response::new(response));
   }