The Oscar-winning actor and Goop CEO shared a photo of herself and Martin standing under a house numbered 42.
“Happy Birthday CAJM,” she wrote.
“This is a special one. We love you so much. #42”
The couple got married in 2003 and in 2013 announced they were “consciously uncoupling”.
Paltrow and Martin have two children together, Apple, 14, and Moses, 12.
The Coldplay front man’s birthday is just one day after Paltrow’s new husband, Brad Falchuk’s.
Paltrow also celebrated Falchuk’s 48th birthday with a heartwarming Instagram post.
“It’s your birthday, but it’s really feels like you are the gift. Not just to me, and to @izzyfalchuk and Brody, but to all of us who are fortunate enough to know and love you, she wrote.
“You are my rock, my true north, my favourite person to eat with, to travel with. You are a true friend. You want only what is for the highest good.
“You always help me see what I can’t. I am so happy you were born @bradfalchuk I love you so much.”
The couple married in the Hamptons in late September 2018 in an intimate ceremony at Paltrow’s house, alongside friends and family.
Paltrow confirmed the news with a sweet shot of the two showing off their wedding bands on Instagram the day after the ceremony.
The Iron Man actor and her ex-husband have remained close since their split.
In January, Paltrow opened up about their relationship explaining that they were “always meant to be together”.
“It doesn’t have to be the end of your relationship with somebody,” she told ES magazine.
“I think Chris and I were meant to be together and have our kids. But our relationship is much better like this: friends and co-parents and family.”
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