WHO is the ex-lover who hooked up JAKE GYLLENHAAL with America’s sweetheart TAYLOR SWIFT?!

Reese still feels so bad about dumping Jake Gyllenhaal that she recently played matchmaker and hooked him up with country cutie Taylor Swift, sources say.

The Brokeback Mountain hunk has been unhappily single since splitting from Reese about a year ago, sources say, while the Oscar-winning actress is seriously dating agent Jim Toth.

"Reese and Jim are deliriously happy," divulged a close pal. "They are living together in Los Angeles and are talking about getting engaged. But Jake hadn’t found anyone else, and has been miserable."

The Legally Blonde beauty stayed in touch with Jake after their two-year romance ended, and Jake, 29, recently confided that Taylor, 20, had caught his eye, said the source.

That’s when Reese went into matchmaking mode, said the source.

She called the young star- who’s dated Twilight hunk Taylor Lautner, heartthrob Joe Jonas and musician John Mayer – to sing her ex’s praises, said the insider.

"Reese told Taylor that Jake’s a good guy who’s romantic and a lot of fun, but would never take advantage of her," said the source.

Taylor then invited Jake to accompany her to New York where her friend Emma Stone was hosting Saturday Night Live, the source revealed.

"Jake and Taylor went to the show, had breakfast the next morning and spent a wonderful weekend together," said the source.

"Reese told a friend, ‘I’ve felt bad that Jake’s been hurting since we broke up.

"Hopefully, Taylor’s the one for him.’"