In a since-deleted post on X, Nicki called out Britney's ex-husband, saying ... "KEVIN FEDERLINE B*TCH WHEN I CATCH U."

Nicki was quote-tweeting Britney's X post about "constant gaslighting" from Kevin that was "extremely hurtful and exhausting."

Kevin's got a tell-all memoir dropping next week -- "You Thought You Knew" -- and the bits and pieces being leaked to the media paint Britney in a bad light.

Among Kevin's claims ... Britney called Justin Timberlake the night before marrying Kevin, Britney made out with a female dancer in a hotel room in Amsterdam, Britney did cocaine while still in breastfeeding phase, and Britney watched their boys sleep with a knife in her hand.

Kevin also came on "TMZ Live" Wednesday and told us the family was alarmed by what they've seen from Britney since her conservatorship ended.