Sources close to the "Don't Be Tardy" star tell TMZ ... Brielle Biermann is dating Michael Kopech, a 20-year-old pitcher in the Boston Red Sox minor league system.
We're told she and Michael started chatting in March, but things started getting serious recently, meaning they're monogamous now ... a big step for 19-year-old Brielle. It's a long distance thing at this point -- she travels to Salem, VA to see his games, and he comes to Atlanta when he can.
Michael's a tall drink at 6'3'' and 205 pounds, and we're told the family loves the fact he's a gentleman.