A singer named MATT NATHANSON is accusing TAYLOR SWIFT of ripping off one of his lyrics.

He believes Taylor's song "All Too Well", from her album "Red" . . . has a line that is almost identical to a line in his song "I Saw", from his 2003 album "Beneath These Fireworks".  And you know what?  They ARE virtually the same.

Here's Taylor's line, quote, "And I forget about you long enough to forget why I needed to."

And here's Nathanson's, quote, "And I'll forget about you long enough to forget why I need to."

By the way, if Nathanson's name sounds familiar, it's because Taylor has, on two different occasions, written his lyrics on her arm while performing.

At a show last year she scribbled, "You're someone else's baby you're someone else's crazy, now" . . . which is from Nathanson's song "Queen of (K)nots".

The second time was earlier this year when she wrote on her arm, quote, "She said watch your back, I'm nobody's girlfriend".  That's from his song "Modern Love".  (--You can see one of those scribblings for yourself here.)

By the way, this all started a couple of days after "Red" came out.  That's when Nathanson Tweeted, quote, "She's definitely a fan . . . and now she's a thief." 

He deleted that post pretty quickly . . . perhaps because Taylor's fans got all huffy and started writing nasty stuff on his Twitter page.

Eventually, he got back into it and started firing back  . . . including this Tweet, quote, "So far, my favorite thing about today is the wave of @taylorswift13 fans calling me a [rhymes with wick] on Twitter for writing 'I Saw' in 2003."