The incident happened Sunday. After finding out what hackers had done, YouTube quickly removed the X-rated material.
Because of the breach, Sesame Street's YouTube channel was unavailable as of Monday morning.
Material for the popular show is expected to be back online in the afternoon.
"Sesame Street" Executive Producer Carol-Lynn Parente said it appears that the X-rated videos were posted for less than an hour and that "Sesame Street" had received no viewer complaints.
The Associated Press contributed to this report.