Jet cornerback Antonio Cromartie mistreated one of his nine young children, smoked pot and dodged detection on his NFL-mandated drug tests, claims an ex-fiancée in a new bombshell memoir about their rocky romance.

In her just-published book, “Love, Intercepted: A Tale of Football, Falling and Failing in Love,” Rhonda Patterson writes she witnessed Cromartie hit his oldest son, now 6, “so hard in the chest that he fell to the floor.”

Patterson, a lawyer and former Miss Black North Carolina, has a two-year-old daughter with Cromartie, his sixth of nine kids he’s sired with eight women.

“I knew that he had smoked marijuana on multiple occasions, but always found a way to avoid the NFL’s mandated drug testing,” she writes.

Patterson also says that Cromartie sips promethezyne, a cough syrup with codeine, which is sometimes used for recreation.

The book is an account of their on-and-off relationship, she told The Post, though she never names Cromartie — referring to him only as “he.”

Cromartie and the Jets declined comment.