diff --git a/lldb/source/Core/IOHandlerCursesGUI.cpp b/lldb/source/Core/IOHandlerCursesGUI.cpp
--- a/lldb/source/Core/IOHandlerCursesGUI.cpp
+++ b/lldb/source/Core/IOHandlerCursesGUI.cpp
@@ -1619,7 +1619,12 @@
   TreeItem(TreeItem *parent, TreeDelegate &delegate, bool might_have_children)
       : m_parent(parent), m_delegate(delegate), m_user_data(nullptr),
         m_identifier(0), m_row_idx(-1), m_children(),
-        m_might_have_children(might_have_children), m_is_expanded(false) {}
+        m_might_have_children(might_have_children), m_is_expanded(false) {
+    // Expand root tree items by default.
+    if (m_parent == nullptr) {
+      m_is_expanded = true;
+    }
+  }
 
   TreeItem &operator=(const TreeItem &rhs) {
     if (this != &rhs) {
@@ -2196,11 +2201,16 @@
         TreeItem t(&item, *m_thread_delegate_sp, false);
         ThreadList &threads = process_sp->GetThreadList();
         std::lock_guard<std::recursive_mutex> guard(threads.GetMutex());
+        ThreadSP selected_thread = threads.GetSelectedThread();
         size_t num_threads = threads.GetSize();
         item.Resize(num_threads, t);
         for (size_t i = 0; i < num_threads; ++i) {
-          item[i].SetIdentifier(threads.GetThreadAtIndex(i)->GetID());
+          ThreadSP thread = threads.GetThreadAtIndex(i);
+          item[i].SetIdentifier(thread->GetID());
           item[i].SetMightHaveChildren(true);
+          if (selected_thread->GetID() == thread->GetID()) {
+            item[i].Expand();
+          }
         }
         return;
       }
diff --git a/lldb/test/API/commands/gui/expand-threads-tree/Makefile b/lldb/test/API/commands/gui/expand-threads-tree/Makefile
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/commands/gui/expand-threads-tree/Makefile
@@ -0,0 +1,3 @@
+C_SOURCES := main.c
+ENABLE_THREADS := YES
+include Makefile.rules
diff --git a/lldb/test/API/commands/gui/expand-threads-tree/TestGuiExpandThreadsTree.py b/lldb/test/API/commands/gui/expand-threads-tree/TestGuiExpandThreadsTree.py
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/commands/gui/expand-threads-tree/TestGuiExpandThreadsTree.py
@@ -0,0 +1,55 @@
+"""
+Test the 'gui' default thread tree expansion.
+The root process tree item and the tree item corresponding to the selected
+thread should be expanded by default.
+"""
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.lldbpexpect import PExpectTest
+
+class TestGuiExpandThreadsTree(PExpectTest):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    # PExpect uses many timeouts internally and doesn't play well
+    # under ASAN on a loaded machine..
+    @skipIfAsan
+    @skipIfCursesSupportMissing
+    def test_gui(self):
+        self.build()
+
+        self.launch(executable=self.getBuildArtifact("a.out"), dimensions=(100,500))
+        self.expect("breakpoint set -r thread_start_routine", substrs=["Breakpoint 1", "address ="])
+        self.expect("run", substrs=["stop reason ="])
+
+        escape_key = chr(27).encode()
+
+        # Start the GUI and close the welcome window.
+        self.child.sendline("gui")
+        self.child.send(escape_key)
+        self.child.expect_exact("Threads")
+
+        # The thread running thread_start_routine should be expanded.
+        self.child.expect_exact("frame #0: thread_start_routine")
+
+        # Exit GUI.
+        self.child.send(escape_key)
+        self.expect_prompt()
+
+        # Select the main thread.
+        self.child.sendline("thread select 1")
+
+        # Start the GUI.
+        self.child.sendline("gui")
+        self.child.expect_exact("Threads")
+
+        # The main thread should be expanded.
+        self.child.expect("frame #\d+: main")
+
+        # Quit the GUI
+        self.child.send(escape_key)
+
+        self.expect_prompt()
+        self.quit()
diff --git a/lldb/test/API/commands/gui/expand-threads-tree/main.c b/lldb/test/API/commands/gui/expand-threads-tree/main.c
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/commands/gui/expand-threads-tree/main.c
@@ -0,0 +1,10 @@
+#include <pthread.h>
+
+void *thread_start_routine(void *arg) { return NULL; }
+
+int main() {
+  pthread_t thread;
+  pthread_create(&thread, NULL, thread_start_routine, NULL);
+  pthread_join(thread, NULL);
+  return 0;
+}