The 27-year-old was arrested on March 2 on suspicion of sexual activity with a 15-year-old girl.
He was bailed until April 23 when he is due to report back to the police.
Johnson was suspended by Sunderland pending the investigation but has today returned to training and could play for new manager Dick Advocaat on Saturday against West Ham.
"We recognise that the player is entitled to re-commence his duties with the club while the legal process continues. He will therefore return to training," a club statement said.
"The club's own investigation cannot continue until the conclusion of the legal process."