Luke Bryan just made official what we told you about early Friday morning -- he and Lionel Richie are joining Katy Perry as judges #2 and 3 on "American Idol."

The country star made the announcement in a video he posted on Twitter just moments ago and hours after TMZ first reported the pending news.

Luke -- all the way from Hayfield, Kansas -- said he's excited to join "the beautiful Ms. Katy Perry and my hero Lionel Richie and my fellow Georgia boy, Ryan Seacrest."

As we reported ... signing all 3 judges didn't come cheap.