The 43-year-old Murder in the First star, who split from wife Idina Menzel last December, was spotted at dinner with Brown in Hollywood back in April.
"They are 100 percent in love," a source told E! News at the time. "They have been dating for a while."
Brown has two children with her ex-husband, former NFL star Ralph Brown. Diggs and Menzel have one son together, 4-year-old Walker.
"Right now, we're still trying to figure out a lot of stuff because we're on different coasts and our son is getting older," Diggs said of his estranged wife earlier this month.