"(Rosario) was telling people a few weeks ago that they're over. She's really upset," a source reported to Us Weekly.
It is unknown why the couple split, but in March, Dawson confided in Britain's Culture magazine that the attention her boyfriend gets from other women can be difficult to handle, Us Weekly reported Thursday.
"Every time we go to a party, Jason is soon surrounded by a group of women of all ages," she said. "They openly put their hands all over him -- even between his legs -- and ask him out. I am sitting there alongside him, watching all this."
She added: "Even I have been surprised by their behavior. Yet I always think it is impossible to shock me."
Lewis, 35, got his acting break as Smith Jerrod on "Sex and the City."
Dawson, 27, just finished shooting "Killshot" and will reprise her role as "Gail" in Sin City 2 next year.