Introduction: Evidence through 2012 suggests that the 2010 Affordable Care Act Dependent Coverage Provision, extending dependent insurance coverage eligibility to age 26years, increased young adult insurance coverage and decreased cancer diagnosis stage in young adult cancer patients. This study examines Dependent Coverage Provision–associated changes in insurance coverage and diagnosis stage through 2014 in young adult cancer patients. Methods: Using a quasi-experimental study design, analyses were conducted in 2017–2018 using 2007 to 2014 data from the Surveillance, Epidemiology, and End Results (SEER) 18 and the National Cancer Database (NCDB). Using difference-in-differences analyses applied to linear probability models, changes in the percentage of policy-eligible individuals aged 19–25years versus ineligible individuals aged 27–29years who were insured (excluding Medicaid) and diagnosed at early (Stages 0 and 1) or late (Stage 4) stages following Dependent Coverage Provision enactment were estimated. Results: A total of 36,901 and 92,358 young adults were included from SEER and NCDB. Consistent increases in the percentage insured (SEER: 3.45 percentage points, 95% CI=2.04, 4.87; NCDB: 3.72 percentage points, 95% CI=2.80, 4.64); variable increases in early-stage diagnoses (2.25 percentage points, 95% CI=0.40, 4.10; 0.69 percentage points, 95% CI= –0.65, 2.02); and decreases in late-stage diagnoses (–1.74 percentage points, 95% CI= –3.10, –0.38; –0.58 percentage points, 95% CI= –1.46, 0.30) were observed in young adults aged 19–25 versus 27–29years. Conclusions: These results provide clear evidence for a Dependent Coverage Provision–associated impact on insurance coverage in young adult cancer patients; however, clear impacts on diagnosis stage are less evident.