"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!," the message, posted on the Sussex Royal account, reads.
The birthday post is accompanied by a collection of photos of Prince Harry throughout his life, starting as a baby in his mother Princess Diana's arms and ending with a previously unseen photo of him with baby Archie and wife Meghan.
The duchess reciprocated a similar post sent by Harry to mark her 38th birthday last month. He posted a message on social media, paying tribute to his "amazing wife."
The message comes a week before the family embarks on a trip to Africa.
Harry and Meghan, accompanied by Archie, will head to South Africa together, while Harry will also visit Angola, Malawi and Botswana between September 23 and October 2.
It will be their first official overseas trip as a family, as Meghan returns from maternity leave, and will focus on "community, grassroots leadership, women's and girls' rights, mental health, HIV/AIDs and the environment."
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