The couple announced their separation last week.
On Friday, Carter shared a photo of herself and Miley Cyrus on a yacht during their vacation in Lake Como, Italy, and her ex took notice.
"Rock the boat, don’t rock the boat baby ⚓️," the blonde beauty, 30, captioned the pic, which shows her wearing a black-and-white striped bathing suit. The "Nothing Breaks Like a Heart" singer is in the background and dons a black Chanel one-piece.
The sexy lake-side snap caught the attention of Jenner, who -- in a since deleted comment -- wrote, "Hot girl summer [flame emoji]."
Jenner's comment comes a week after he and Carter announced their separation after five years together.
"Brody Jenner and Kaitlynn Carter have decided to amicably separate," the former couple's reps confirmed in a statement to ET. "They love and respect one another, and know that this is the best decision for their relationship moving forward."
The two got married in Bali in June 2018. However, a source close to the couple told ET that the two were never legally married.
"They never did it in the U.S., so it wasn’t legal. But they had a wedding in Indonesia," the source said, adding that Jenner and Carter had been having problems for some time.
For more on their split, watch below.
Brandon Lee Reacts to Brody Jenner and Kaitlynn Carter's Breakup: 'It's Devastating' (Exclusive)