The witness provided graphic testimony at the pop star's trial Thursday, saying the alleged incident involved a boy who later reached a financial settlement with the entertainer.
The former guard alleges the abuse occurred at the singer's Neverland Ranch.
In 1994, Mr. Jackson paid a multimillion-dollar settlement to a teenage boy, and no charges of abuse were filed.
Two weeks ago, the judge presiding over Mr. Jackson's trial said testimony about alleged prior offenses could be introduced as evidence. Prosecutors say such allegations reveal a pattern of abuse.
Mr. Jackson has denied all charges against him.
Some information for this story provided by AP and AFP.