diff --git a/README.md b/README.md
index 049aeb3..1ff502e 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,7 @@ This private repository is a hands-on sandbox for learning pull requests.
 ```bash
 python3 task_tracker.py list
 python3 task_tracker.py add "Review a pull request"
+python3 task_tracker.py complete 1
 ```
 
 ## Run tests
diff --git a/task_tracker.py b/task_tracker.py
index e6de844..819d502 100644
--- a/task_tracker.py
+++ b/task_tracker.py
@@ -23,15 +23,27 @@ def add_task(tasks, title):
     return tasks
 
 
+def complete_task(tasks, task_number):
+    if task_number < 1 or task_number > len(tasks):
+        raise ValueError("task number is out of range")
+    tasks[task_number - 1]["done"] = True
+    return tasks
+
+
 def render_tasks(tasks):
     if not tasks:
         return "No tasks yet."
-    return "\n".join(f"- [{'x' if task['done'] else ' '}] {task['title']}" for task in tasks)
+    return "\n".join(
+        f"- [{'x' if task['done'] else ' '}] {task['title']}"
+        for task in tasks
+    )
 
 
 def main():
-    parser = argparse.ArgumentParser(description="A tiny task tracker for PR practice")
-    parser.add_argument("command", choices=["list", "add"])
+    parser = argparse.ArgumentParser(
+        description="A tiny task tracker for PR practice"
+    )
+    parser.add_argument("command", choices=["list", "add", "complete"])
     parser.add_argument("value", nargs="?")
     args = parser.parse_args()
 
@@ -44,7 +56,16 @@ def main():
             parser.error(str(error))
         save_tasks(tasks)
         print(f"Added: {args.value}")
+    elif args.command == "complete":
+        if args.value is None or not args.value.isdigit():
+            parser.error("task number must be a positive integer")
+        try:
+            tasks = complete_task(load_tasks(), int(args.value))
+        except ValueError as error:
+            parser.error(str(error))
+        save_tasks(tasks)
+        print(f"Completed task {args.value}")
 
 
 if __name__ == "__main__":
-    main()
+    main()
\ No newline at end of file
diff --git a/tests/test_task_tracker.py b/tests/test_task_tracker.py
index 8d2ac5e..608e654 100644
--- a/tests/test_task_tracker.py
+++ b/tests/test_task_tracker.py
@@ -2,7 +2,13 @@
 import unittest
 from pathlib import Path
 
-from task_tracker import add_task, load_tasks, render_tasks, save_tasks
+from task_tracker import (
+    add_task,
+    complete_task,
+    load_tasks,
+    render_tasks,
+    save_tasks,
+)
 
 
 class TaskTrackerTests(unittest.TestCase):
@@ -27,9 +33,30 @@ def test_add_and_save_task(self):
             )
 
     def test_add_task_rejects_blank_title(self):
-        with self.assertRaisesRegex(ValueError, "task title cannot be empty"):
+        with self.assertRaisesRegex(
+            ValueError,
+            "task title cannot be empty",
+        ):
             add_task([], "   ")
 
+    def test_complete_task_marks_selected_task_done(self):
+        tasks = [
+            {"title": "First task", "done": False},
+            {"title": "Second task", "done": False},
+        ]
+
+        result = complete_task(tasks, 2)
+
+        self.assertFalse(result[0]["done"])
+        self.assertTrue(result[1]["done"])
+
+    def test_complete_task_rejects_out_of_range_number(self):
+        with self.assertRaisesRegex(
+            ValueError,
+            "task number is out of range",
+        ):
+            complete_task([], 1)
+
 
 if __name__ == "__main__":
     unittest.main()