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.