Norovirus continues to pose a significant burden on cruise ships, causing an average of 27 confirmed outbreaks annually over the past 5 years. In January 2009, the report of a suspected norovirus outbreak among passengers on a cruise ship prompted an investigation. A retrospective cohort study among passengers was conducted on board the ship. Questionnaires about health care-seeking behaviors, hygiene practices, and possible norovirus exposures were placed in every cabin. Stool samples from several ill passengers were tested for norovirus by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR) and confirmed by sequence analysis. Of 1842 passengers, 1532 (83.2%) returned questionnaires, and 236 (15.4% of participants) met the case definition. Of these, 95 (40%) did not report to the infirmary. Case passengers were significantly more likely to have an ill cabin mate (relative risk [RR] = 3.0; P < .01) and to have witnessed vomiting during boarding (RR = 2.8; P = .01). Over 90% of all passengers reported increased hand hygiene practices following the outbreak; 38% of ill passengers and 11% of well passengers decreased participation in public activities. Of 14 samples tested, 12 were positive for norovirus by RT-qPCR; 5 of these were confirmed by sequence analysis and typed as GII.4 Minerva. Person-to-person transmission, including an incident of public vomiting during boarding, likely contributed to this high morbidity outbreak. Infirmary surveillance detected only 60% of acute gastroenteritis (AGE) cases involved in this outbreak. Adjustments to outbreak reporting thresholds may be needed to account for incomplete voluntary AGE reporting and to more rapidly implement control measures.