The Centers for Disease Control in Atlanta said 281 passengers and 22 crew members reported being ill during a cruise that began Jan. 21 and is to run through Jan. 31.
There were 3,050 passengers and 1,165 crew members onboard when the outbreak occurred.
The CDC said patients experienced vomiting and diarrhea. The cause of the illness had not been identified late Saturday.
The CDC said Royal Caribbean Cruise Line and the Explorer of the Seas crew increased "cleaning and disinfection procedures according to their outbreak prevention and response plan," notified passengers of the outbreak and urged them to report any illnesses.
The cruise line dispatched additional crew to help with the cleanup.
Royal Caribbean International spokeswoman Janet Diaz told CNN the ship was subjected to an extensive sanitizing at San Juan, Puerto Rico.
A CDC health officer and an epidemiologist were to board the Explorer of the Seas Sunday to evaluate the outbreak and the cruise line's response.