Background: Medicaid expansion under the Affordable Care Act (ACA) is associated with increased insurance coverage and early stage at diagnosis among patients with cancer. Whether these gains translate to improved survival is largely unknown, however. This study examines changes in one-year survival rates among persons newly diagnosed with cancer following the ACA Medicaid expansion. Methods: Patients aged 18-62 years from 41 population-based state cancer registries diagnosed pre-(2010-2012) and post-(2014-2016) ACA Medicaid expansion were followed through October 1, 2013 and December 31, 2016, respectively. Difference-in-differences (DD) analysis was conducted to estimate changes in one-year overall and cause-specific survival rates associated with Medicaid expansion, adjusting for age group, sex, race/ethnicity, area-level poverty, urban/rural status and region. Stratified analysis was conducted by cancer type, sex and area-level poverty. Results: A total of 2,537,818 patients diagnosed with cancer were included from Medicaid expansion (N = 1,492,729) and non-expansion (N = 1,045,089) states. During follow-up, 291,854 patients died including 246,660 deaths from cancer. The one-year overall survival rate (%) increased from 88.1 pre-ACA to 89.1 post-ACA in Medicaid expansion states and from 85.6 to 86.4 in non-expansion states for both sexes combined, resulting in a net increase of 0.4 (95%CI = 0.3-0.6) in expansion states after adjusting for sociodemographic factors. By cancer site and for both sexes combined, the increase in adjusted one-year overall survival in expansion states versus non-expansion states was greater for cancers of lung (DD = 1.6; 95%CI = 0.8-2.5), pancreas (DD = 2.2; 95%CI = 0.5-3.9) and liver (DD = 3.4; 95%CI = 1.7-5.1); as were the increases for cervix (DD = 1.3; 95%CI = 0.1-2.5), melanoma (DD = 0.4;95%CI = 0.04-0.9), non-Hodgkin lymphoma (DD = 1.2; 95%CI = 0.1-2.2) and esophagus (DD = 7.1; 95%CI = 0.5-13.7) among women. The improvement in one-year overall survival was larger among patients residing in the poorest areas (DD = 0.7; 95%CI = 0.3-1.1) compared to those in the richest areas (DD = 0.2; 95%CI = -0.2 to 0.5), leading to a narrowing survival disparity by area-level poverty. Patterns in one-year cause-specific survival were similar. Conclusions: Medicaid expansion was associated with greater increase in one-year overall survival rates, largely driven by the improvements in survival for cancer types with poor prognosis, suggesting improved access to timely and effective treatments. Furthermore, the increase was largest in poorest areas, highlighting the promising role of Medicaid expansion in reducing health disparities. Future studies should monitor changes in longer-term health outcomes following the ACA.