Chris Bosh to Ex -- GIMME OUR KID ... You're a Bad Mom

Newly-minted NBA champion Chris Bosh has been busy with more than just basketball ... he's been embroiled in a bitter legal battle with his ex to snag custody of their 4-year-old daughter, TMZ has learned.

Bosh has filed legal docs asking a judge to strip his ex, Allison Mathis, of primary physical custody of Trinity.

Bosh claims Mathis has been trying to alienate the kid from him ... blocking contact, refusing to let them travel together, and poisoning their daughter against him.

Bosh wants primary physical custody, but says he's more than willing to give Mathis her fair share of mother-daughter time in the form of visitation.

Mathis has fired back in her own legal docs, saying his complaints are BS.

The custody battle between Bosh and Mathis has been raging for years -- you'll recall, just last June Bosh was denied permission to take his daughter to the Olympics.