The pop star did an about-face on her IG account, posting a screenshot of JT with Jimmy Fallon during his 'Tonight Show' performance from last week -- where they were singing one of his new songs from his latest album -- and singing his praises.

Before she got to the compliments, though, she said sorry in no uncertain terms -- writing, "I wanna apologize for some of the things I wrote about in my book. If I offended any of the people I genuinely care about I am deeply sorry."

BS then hyped up his new singles, adding ... "I also wanted to say I am in love with Justin Timberlake's new song 'Selfish.' It is soo good and how come every time I see Justin and Jimmy together I laugh so hard???" She also shouted out his other songs, 'Sanctified.'

Like we said, it's definitely a sea change from how she spoke about her ex in her memoir, "The Woman In Me," from last fall -- where she said she'd gotten pregnant with him ... but then suggested she got an abortion at his behest, among other allegations against JT.

Britney's fan base has had their knives out for Justin in the wake of all that -- even going so far as to troll his new music by making one of her old songs chart above his on iTunes.

Now, however, Britney is speaking for herself ... and it sounds like she's got regrets -- and maybe some love remaining for her former boyfriend. It certainly sounds like an olive branch.