The “Medicaid undercount” refers to the discrepancy between administrative counts of Medicaid enrollment and estimates from survey data. This paper reports data from a collaboration between the State Health Access Data Assistance Center (SHADAC) and the U.S. Census Bureau to expand previous research on the Medicaid undercount to the American Community Survey (ACS). We use a unique version of the 2009 ACS which has been merged to Medicaid and expansion Children’s Health Insurance Program (CHIP) (referred to as Medicaid Plus) enrollment data from the 2009 Medicaid Statistical Information System (MSIS) to quantify the Medicaid undercount in the ACS. Using explicit survey responses we find that 77.1% of respondents accurately report Medicaid Plus coverage, which represents an undercount of 22.9%. Of those known to have Medicaid Plus 13.2% indicate another type of public or private coverage and 9.6% report that they are uninsured. Taking ACS edit routines into account (the production/ public use version of the data) reduces the undercount to 21.6%, however, we cannot ascertain how many cases the edit routine assigns to Medicaid Plus that do not actually have Medicaid, CHIP, or are enrolled in a state specific coverage plan not reported in the MSIS. Consistent with past research, reporting of coverage is more accurate for children and individuals with lower income and education. The undercount varies across states from a high of 28.4% in Nevada to a low of 13.4% in Vermont.  After adjusting for misreported cases, the overall uninsured rate is reduced by 1.2 percentage points. Although there is some upward bias in estimate of uninsurance, the bias is modest and consistent with results from other national surveys that measure health coverage. The level of accuracy in Medicaid Plus reporting, particularly for children and low income families, should provide policymakers confidence in using the ACS data to evaluate the impact of the Affordable Care Act at the national and state level.