Chris Brown -- Baby Mama's BF ... Royalty's With Me Now, Permanently

Chris Brown's got a war on his hands -- his baby mama's boyfriend just staked his claim to baby Royalty with two new tattoos.

Nia Guzman's BF, King Ba keeps posting adorable pics of himself with Royalty ... and in the latest one he's flaunting fresh ink of a giant royal crown over the name, "Nia."

The tatts are definitely brand new -- because when he was photographed at Easter ... again with Royalty ... the left forearm was clean.

KB seems to be responding to Chris' Instagram last week with Royalty ... where he said, "God has blessed me with my twin."

It's gonna be interesting. Chris has DNA on his side, but as long as Royalty's in Texas -- King's got face time on his side.