From July to September 2019, cruise line X experienced sudden, unexplained outbreaks (>3% of the passenger population) of acute gastroenteritis (AGE) among passengers on 10 cruise ships sailing in Europe. The rapid onset of vomiting and diarrhea followed by recovery within 24 hours were consistent with norovirus infection. Investigations by the cruise line throughout the summer yielded no clear source of the outbreaks even after extensive food testing. On September 18, 2019, CDC’s Vessel Sanitation Program (VSP) was notified of an outbreak of AGE on cruise ship A of cruise line X, sailing into U.S. jurisdiction (defined as passenger vessels carrying ≥13 passengers sailing to the United States from a foreign port) from Germany to New York City (1). By the end of the 19-day voyage on September 23, a total of 117 of 2,046 (5.7%) passengers and eight of 610 (1.3%) crew members met the case definition for AGE (three or more loose stools within a 24-hour period or more than normal for the patient, or vomiting plus one other sign or symptom including fever, diarrhea, bloody stool, myalgia, abdominal cramps, or headache).