He's going from geeky teen to big daddy.
In keeping with Hollywood's tradition for bestowing unusual baby names, the couple named the little one Wylda Rae after she entered the world Wednesday in London.
Johnson, 20, and Wood, 43, first hooked up after she directed him in her 2009 John Lennon biopic, Nowhere Boy, which saw the English actor playing the late Beatle. The pair got engaged on Halloween 2009, but didn't publicly disclose the true nature of their relationship until earlier this year.
Wylda is their first child, though Wood has two daughters from a previous marriage.