"Your service to the causes you care so deeply for inspires me every day, You are the best husband and most amazing dad to our son. We love you. Happiest birthday!" Markle wrote.
The message accompanied a collage of photos from Harry's childhood, including one image of him as a baby with his late mother, Princess Diana.
Other photos show him doing humanitarian work and dressed in uniform during his service in the British Army and the Army Air Corps.