DiCaprio appeared at a birthday party for Bundchen last week, setting off speculation that the two were back together, Us Weekly reported.
Although the pair called off their five-year relationship in November, they began "hooking up again about a month ago," one of Bundchen's friends said. DiCaprio, 31, and Bundchen, 26, "never really got over each other," the source said.
DiCaprio has been dating model Bar Refaeli, 21, since December. Refaeli's mother Zipi, said the couple's relationship is intact.
"They're fine," she said.