A prospective matched case-control study was conducted to evaluate associations between dietary histories, including consumption of bivalves, diarrhea, and norovirus positive diarrhea in adult ambulatory patients at an outpatient clinic of a hospital in Tokyo, Japan. Ambulatory cases with diarrhea were matched with nondiarrheal control patients, who visited the same clinic. A standardized questionnaire was used to obtain patients’ information, including histories of food consumption and clinical information. Norovirus infection was confirmed using real-time reverse transcription polymerase chain reaction. A total of 207 patients, including 69 diarrheal cases and 138 nondiarrheal cases were included in the analysis. Among them, 60 (29.0%) participants reported consuming bivalves. Norovirus was detected in 35% (24/69) of diarrheal cases. Of those, 10 (41.7%) reported consumption of bivalves and of those, 6 (60.0%) consumed raw bivalves. The proportion of those who consumed raw bivalves was significantly higher in norovirus-positive diarrheal cases than in norovirus-negative diarrheal cases (25.0% vs 6.7%; odds ratio [OR], 4.67; 95% confidence interval [CI], 1.1-20.7) and matched nondiarrheal controls (25.0% vs 6.3%, OR: 5.00; 95% CI, 1.1-22.2). The attributable fraction of consuming raw bivalves for norovirus-associated diarrhea to matched nondiarrheal controls was 20.0%. Consuming raw bivalves was substantially attributed to norovirus-associated diarrhea in adult ambulatory patients and preventive measures for reducing the risk associated with consumption of raw bivalves could decrease the incidence of norovirus-associated diarrhea.