SELENA GOMEZ is making on-again, off-again boy­friend Justin Bieber jealous – by dating his best friend!

The troubled young stars called it quits last month, and Selena showed up at a Feb. 10 post-Grammy party at Holly-wood’s Chateau Marmont with Justin’s close pal, 24-year-old video director Alfredo Flores.

“Selena knew Justin was licking his wounds after getting shut out of the Grammys this year, so she took advantage of his vulnerability by looking stunningly sexy with Alfredo on her arm,” an insider told The ENQUIRER.

“It was Selena’s way OF telling Justin there are plenty of fish in the sea. She has no intention of sitting home alone while he tries to make up his mind if he wants to be with her exclusively, or take his chances playing the field.”

Justin, 18, and Selena, 20, dated for two years but hit choppy waters following reports that the “Baby” singer had been romancing other women.

As for Alfredo, he and Justin are such good friends that the two often stay under the same roof when they’re both in Los Angeles.

“Alfredo has a history with Selena too,” the source noted. “The three of them used to hang out together all the time, but that was before the trouble started between Justin and Selena.”

Alfredo has helped out with Selena’s music videos and even traveled to see her recent shows in Europe and Puerto Rico.

“He’s always there when Selena needs him,” added the source. “Now Justin’s fuming that his best friend and his best girl may be cooking up a romance.”