According to a friend close to the situation ... Taylor is in fact "single" again, although it's still unclear exactly why they've now split up.

It was a pretty whirlwind romance ... just last month Matty popped up at all 3 of Tay Tay's Nashville concerts, right on the heels of news she and Joe Alwyn had split after 6 years together.

From then on Matty and Taylor were pretty inseparable when she wasn't performing. We saw them out in NYC multiple times -- double dating with her friends, and hitting up a recording studio.

Matty was even spotted sneakin' into Taylor's NYC townhouse, and he hung out with Taylor's dad during her Eras Tour stop in Philly.

Although they just started dating, the artists have known each other since 2014 ... when Taylor went to a concert for his band, The 1975.

They haven't been seen in public together since May 25 when they were out on a dinner date.

We still don't know exactly when the breakup went down, but there was a big clue things were over when The 1975 performed this weekend in Denmark.

Matty had a history of kissing random people in the crowd, but during a January concert -- which Taylor was attending -- he said he was putting the kibosh on that move.

Well, lo and behold, during this weekend's concert he locked lips with a security guard. Talk about single and ready to mingle!!!

Of course, that goes for Taylor too.

We covered it all in the latest episode of The TMZ Podcast, available on all podcast platforms.