Adam Levine Says No Affair with IG Model, but He 'Crossed the Line'

Adam Levine says the IG model who claims she was manipulated into having a year-long physical relationship with him is full of it ... though he admits he "crossed the line."

Adam tells TMZ, "I used poor judgment in speaking with anyone other than my wife in ANY kind of flirtatious manner. I did not have an affair, nevertheless, I crossed the line during a regrettable period of my life."

He goes on ... "In certain instances it became inappropriate. I have addressed that and taken proactive steps to remedy this with my family."

And he says ... "My wife and my family is all I care about in this world. To be this naive and stupid enough to risk the only thing that truly matters to me was the greatest mistake I could ever make. I will never make it again. I take full responsibility. We will get through it and we will get through it together."

Sumner Stroh laid it all out on TikTok Monday morning -- just a few days after Adam's wife, Victoria's Secret model Behati Prinsloo, announced their 3rd pregnancy.

Sumner claimed she was tricked into having an affair with the singer ... saying it happened at a point when she was young and naive -- adding she now feels exploited.

She even showed alleged screenshots where he messaged her after months of radio silence ... writing, "Ok serious question. I’m having another baby and if it’s [a] boy I really wanna name it Sumner. You ok with that? DEAD serious."

The Internet blew up over the claims ... with Adam trending on Twitter. Some were buying it, while others suspect Sumner of chasing clout.

Adam and Behati have been together since 2012 -- they did split briefly in May 2013, but announced their engagement a couple months later.