Out-of-network emergency department (ED) use, or use that occurs outside the contracted network, may lead to increased care fragmentation and cost. We examined factors associated with out-of-network ED use among Medicaid beneficiaries.Enrollment, claims, and encounter data for adult Medi-Cal health plan members with 1+ ED visits and complete Medicaid eligibility during the study period from 2013 to 2014.We analyzed the data to identify factors associated with out-of-network ED use classified by mode of arrival (ambulance vs. nonambulance).We extracted encounter, ambulance, and ED census data and linked them together based on ED visit date.Of 11,143 ED visits, 6,808 (61.1 percent) were out-of-network. The number of hours the study ED was on ambulance diversion increased the odds of out-of-network visits for the 3,365 (30.2 percent) ED visits arriving by ambulance. For all visit types, assignment to a primary care clinic at the in-network hospital and having had any primary care visit during the study period decreased the odds of out-of-network ED care. Individuals were more likely to go out-of-network for ED care if they lived in neighborhoods containing out-of-network EDs.There are a number of factors related to out-of-network ED use, including the proximity and density of out-of-network EDs, race and ethnicity, a prior history of out-of-network ED use, and individuals' connection to primary care. EDs that serve Medicaid beneficiaries may need to explore alternative sites and modalities of care as alternatives to the ED, and consider their ability to absorb large numbers of out-of-network visits given already limited capacity.