Background: One of the proposed benefits of expanding insurance coverage under the Affordable Care Act (ACA) was a reduction in emergency department (ED) utilization for non-urgent visits related to lack of health insurance coverage and access to primary care providers. The objective of this study was to estimate the effect of the 2014 ACA implementation on ED use in New York. Methods: We used the Healthcare Cost and Utilization Project State Emergency Department and State Inpatient Databases for all outpatient and all inpatient visits for patients admitted through an ED from 2011 to 2016. We focused on in-state residents aged 18 to 64, who were covered under Medicaid, private insurance, or were uninsured prior to the 2014 expansion. We estimated the effect of the expanded insurance coverage on average monthly ED visits volumes and visits per 1000 residents (rates) using interrupted time-series regression analyses. Results: After ACA implementation, overall average monthly ED visits increased by around 3.0%, both in volume (9362; 95% Confidence Intervals [CI]: 1681–17,522) and in rates (0.80, 95% CI:0.12–1.49). Medicaid covered ED visits volume increased by 23,972 visits (95% CI: 16,240 –31,704) while ED visits by the uninsured declined by 13,297 (95% CI:−15,856 – −10,737), and by 1453 (95% CI:-4027–1121) for the privately insured. Medicaid ED visits rates per 1000 residents increased by 0.77 (95% CI:-1.96–3.51) and by 2.18 (95% CI:-0.55–4.92) for those remaining uninsured, while private insurance visits rates decreased by 0.48 (95% CI:-0.79 – -0.18). We observed increases in primary-care treatable ED visits and in visits related to mental health and alcohol disorders, substance use, diabetes, and hypertension. All estimated changes in monthly ED visits after the expansion were statistically significant, except for ED visit rates among Medicaid beneficiaries. Conclusion: Net ED visits by adults 18 to 64 years of age increased in New York after the implementation of the ACA. Large increases in ED use by Medicaid beneficiaries were partially offset by reductions among the uninsured and those with private coverage. Our results suggest that efforts to expand health insurance coverage only will be unlikely to reverse the increase in ED use.