The 46-year-old Bad Boy Records boss celebrated Cassie turning 30 on Friday with a photo montage on Instagram, reported Ace Showbiz.
"Happy birthday baby! @cassie I love you always. God bless you," he wrote, alongside several heart emojis.
The public declaration of love emerged days after reports suggested Diddy and Cassie had parted ways following a heated argument last week, when police were called to investigate.
The stars have yet to comment on the fight or the break-up rumours, which came at a tough time for Diddy as he recovers from rotator cuff surgery on his shoulder.