THE DNA TEST IS IN -- Michael Lohan fathered an illegitimate lovechild back in 1995 behind Dina Lohan's back ... and Michael's reaction to the shocking paternity reveal was all caught on tape.

Michael agreed to submit to a DNA test for an episode of "Trisha Goddard" -- where he comes face-to-face for the first time with the 17-year-old girl who claimed to be his daughter -- and the results came back POSITIVE.

The girl's mother -- Kristi Horn -- has claimed Michael was the father for years, and has even tried multiple times to get him to pay child support ... but Michael adamantly denied the allegations.

The reveal -- which airs tonight -- is cringeworthy ... Michael can't believe the news ... and the girl's mother immediately bursts into tears. 

After, Michael gets up to hug his newfound daughter (Ashley) and she resists, shouting, "No! No! No!" while Michael pleads, "I just want to hug you."

FYI ... Michael's two youngest children with Dina -- Ali and Cody -- were born in 1993 and 1996 respectively. Which means Ashley was conceived somewhere in the middle.

Congrats, Lindsay. You got a new half-sister.

Mark this day for the history books ... Michael Lohan has gone on record to tell TMZ ... "I'LL NEVER LEAVE KATE MAJOR."

TMZ caught up with MiLo at LAX  last night and we asked him about his alleged baby mama Kristi Horn ... who recently ripped him a new one​ and called him an absentee father who owes hundreds-of-thousands in back child support.

MiLo explained his side of the story (here's a hint: DENY, DENY, DENY) but more importantly, he took the opportunity to defend his relationship with Kate and tell us he's in it for the long haul.

Talk about JINXING it.

Once a deadbeat dad, ALWAYS a deadbeat dad ... so says one of Michael Lohan's baby mamas ... who says it's a lesson Kate Major is about to learn the hard way.

TMZ spoke with Kristi Horn - who has a 17-year-old daughter named Ashley with Lohan. Horn tells us Michael isn't just an absentee father ... he also owes her more than $300k in back child support.

And Horn says there's a ZERO PERCENT chance MiLo changes his ways with the new kid on the way.

 "This is a freaking train wreck ... here we go again," Horn says.

"I just feel so bad for Kate. She’s going to be a single mother eventually ... he’s going to leave her to raise their child alone like he did to Dina and I."

Maybe, maybe not ... but probably maybe.