Barack Obama has shared a throwback picture of him and Michelle Obama on Instagram to hounour his wife's 55th birthday.

The photo of the couple taken in Kenya in their younger days took fans down memory lane, prompting an outpouring of touching messages to honour the former first lady.

The former president posted a sweet message with the image, saying: "I knew it way back then and I'm absolutely convinced of it today — you're one of a kind, @MichelleObama. Happy Birthday!"

The post has over 4.5 million likes on Instagram and another million on Twitter at the time of writing.

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The photo appears to be from the early 1990s, taken soon after they were engaged.

The couple, who met in the late 1980s at a Chicago law firm, tied the knot in 1992.

Michelle took to Instagram sharing a snap of of flowers with the caption: "Thank you all so much for the birthday love—I love you all right back!

"Feeling so incredibly thankful for my South Side roots, my soul-affirming partner and daughters, and every unimaginable twist and turn over these 55 years. Can't wait to see what becomes of the next one!"

Among those wishing Michelle a happy birthday was Beyonce, who posted a childhood photo of the former FLOTUS with 'BOW DOWN' written in all caps, the Daily Mail reported.

Writer Shonda Rhimes sent a tweet, saying: "Happy birthday @michellobama. To many more years of friendship!"

Rhimes, who is the creator of hit shows Grey's Anatomy, Private Practice and Scandal, also shared a photo of the pair together at a Conference for Women event. Both women grew up in Chicago.

Congresswoman Kamala Harris also wished the former FLOTUS well for her 55th birthday.

"Happy birthday, @MichelleObama!' the California Democratic Rep tweeted, along with a picture of the two of them. 'Thank you for being a tireless advocate for our nation's youth and an inspiration to so many across the country."

Celebrity TV chef Rachael Ray also honoured the former first lady.

It's not clear exactly how Michelle is celebrating her birthday this year as she's remained quiet on social media throughout the day.

However, it may well be spent relaxing as she's in the midst of a huge book tour to promote her memoir Becoming, which has sold over 3million copies.