For adults with a complaint of diarrhea presenting to United States emergency departments (EDs) from 2016 to 2021, we examined the: (1) occurrence and temporal trends in these ED visits, (2) frequency with which services were provided (laboratory testing, radiologic imaging, and intravenous fluids (IV fluids) administration) and patients were admitted; and (3) factors associated with service provision and admission. Data from the National Hospital Ambulatory Medical Care Survey (2016-2021) were analyzed. Multivariable logistic regression modeling was employed to examine factors associated with service provision and admission, according to patient demographic characteristics, healthcare insurance status, and associated clinical symptoms; ED geographic location; and type of ED medical staff who evaluated the patient. From 2016 to 2017, there were 3.3-3.7 million ED visits/year by adults with a complaint of diarrhea (3.1% [95% CI 2.9-3.3] of all adult US ED visits). Services were provided and patients were admitted per these frequencies: complete blood count (80%; 95% CI 76-83); blood culture (8%; 95% CI 6-9); metabolic panel (94%; 95% CI 86-97); ultrasound (8%; 95% CI 7-10); abdominal/pelvic CT (33%; 95% CI 29-35); IV fluids (63%; 95% CI 50-66); and admission (16%; 95% CI 14-18). Factors associated with receipt of these services and admission included other presenting symptoms (abdominal pain, vomiting, and nausea), ED geographic location, ED medical staff member evaluating the patient, race, Hispanic ethnicity, and type of health insurance. For adult patients presenting to US EDs with a complaint of diarrhea, US EDs highly utilized selected laboratory tests and radiologic imaging. Differences in utilization raise concerns about equitable healthcare delivery and call for further investigation into the underlying reasons, as well as the development and adoption of standardized care pathways.