The 42-year-old dad-of-five shared a photo of himself with his arm round his son during a walk.
“Happy 15th birthday big boy,” he wrote on Instagram on Friday 1 September.
“Fine young man growing up to be a fine young gentleman - but has only beaten his dad at tennis just the once so far.
“But maybe now he is 15 that’s gonna change.”
Victoria Beckham also sent birthday wishes to her son on Instagram.
She shared a photo of herself with her arm around Romeo and wrote: “Kisses birthday boy. We all love you so, so much Romeo. 15 today!”
The couple are also parents to Brooklyn, 18, Cruz, 12, and Harper, six.