The 28-year-old -- who played for Sunderland in the English Premier League -- was accused of hooking up with the girl in his Range Rover back in January 2015.
The girl told cops Johnson put his hands down her pants and she performed oral sex on him. Johnson was cleared of the oral sex charge ... but found guilty of digitally penetrating the child.
Today in the court ... the judge told Johnson, "I am satisfied that the girl suffered serious psychological harm."
After the sentence, one of the detectives who worked on the case said, "Fame, celebrity and a position of power does not give you the right to break the law in pursuit of whatever you desire.