"Happy birthday Barack!" she wrote. "Your hair's a little grayer, but I love you more than ever."
Her tweet was accompanied by a sweet black and white photo of the Obamas as a young couple, relaxing together on a sofa.
The US leader spent his birthday weekend at Camp David, the rural presidential retreat in Maryland. While there, he played golf with some of his oldest friends, from both Hawaii and Chicago, before returning to Washington on Sunday.
This week, he is due to travel to the West Coast, where he will visit troops at Camp Pendleton and also appear on The Tonight Show with Jay Leno. It will be his sixth appearance on the hit chat show, and his fourth since being elected.
Barack and Michelle have been happily married for 20 years now, and are proud parents to two daughters, Malia, 15, and Sasha, 12.
Michelle took a trip down memory lane earlier this month when she shared an old family photo with her followers. The picture, titled "Fab four #Throwback Thursday" shows the Obamas proudly posing with their two girls, when they were still both very young.
Of course, Malia and Sasha have spent much of their childhood living in the White House. Given their unusual upbringing, the girls are constantly reminded that they are growing up in a 'slightly unreal environment', their father recently revealed.
In an interview published last week on Amazon's Kindle Singles, the President lamented society's focus on celebrity lifestyles, saying that when he was growing up, "Kids weren't monitoring every day what Kim Kardashian was wearing, or where Kanye West was going on vacation, and thinking that somehow that was the mark of success."
He went on to say that Malia and Sasha have family members and friends who are less fortunate than themselves, and so understand that their lifestyle is "not the norm".