Background For parents, a diagnosis of cancer in their child is a traumatic experience. However, there is conflicting evidence about the risk of developing mental illness among parents following diagnosis. Our objective was to conduct a meta‐analysis to determine the prevalence of mental illness in parents of children with cancer. Methods Four databases were searched to identify articles describing the prevalence of anxiety, depression, or posttraumatic stress disorder (PTSD) in parents of pediatric cancer patients. Two reviewers independently screened and extracted data. Subgroup analyses by gender and phase of cancer experience were selected a priori. Studies were reviewed in accordance with PRISMA guidelines. Results Of 11 394 articles identified, 58 met inclusion criteria. Reported prevalence was highly heterogeneous, ranging from 5% to 65% for anxiety (pooled prevalence 21% 95% CI, 13%–35%), 7% to 91% for depression (pooled prevalence 28% 95% CI, 23%–35%), and 4% to 75% for PTSD (pooled prevalence 26% 95% CI, 22%–32%). Prevalence was consistently higher than noncancer parental controls. Heterogeneity was not explained by parental gender or child's cancer phase and was instead likely due to significant methodological differences in measurement tools and defined thresholds. Conclusions Parents of children with cancer have a higher prevalence of anxiety, depression, and PTSD compared with population controls. Yet, the reported prevalence of mental illness was highly variable, hampering any conclusive findings on absolute prevalence. To better understand the risk of long‐term mental illness in this population and target interventions, future studies must adhere to standardized reporting and methods.