Many patients with cancer do not gain Medicaid coverage until a cancer diagnosis, which can reduce access to early cancer detection and timely treatment, potentially driving inferior survival. Little is known about whether continuous Medicaid coverage prediagnosis through postdiagnosis (v gaining Medicaid at/after diagnosis) provides survival benefits for pediatric/adolescent oncology patients. We identified patients newly diagnosed with cancer at age 21 years or younger in a large pediatric health system between 2007 and 2016. Electronic medical records (EMRs) were linked to Medicaid administrative data to differentiate insurance continuity patterns during the 6 months preceding through the 6 months after cancer diagnosis (assessment window): continuous Medicaid, newly gained Medicaid (at or after diagnosis), and other Medicaid enrollment patterns. For patients not linked to Medicaid data, we used EMR-reported insurance types at diagnosis. We followed patients from 6 months postdiagnosis up to 5 years, death, or December 2020, whichever came first. Multivariable regressions estimated all-cause and cancer-specific survival, controlling for sociodemographic and cancer-related factors. Among 1,800 patients included in the analysis, 1,293 (71.8%) had some Medicaid enrollment during the assessment window; among them, 47.6% had continuous Medicaid and 36.3% had newly gained Medicaid. Patients not linked with Medicaid data had private (26.9%) or other/no insurance (1.2%) at diagnosis. Compared with patients with continuous Medicaid, those with newly gained Medicaid had higher risks of all-cause death (hazard ratio [HR], 1.41 [95% CI, 1.10 to 1.81]; P = .008) and cancer-specific death (HR, 1.46 [95% CI, 1.12 to 1.90]; P = .005). Continuous Medicaid coverage throughout cancer diagnosis is associated with survival benefits for pediatric/adolescent patients. This finding has critical implications as millions of American individuals have been losing coverage since the unwinding of the Medicaid Continuous Enrollment Provision.