Background The psychological toll on parents of a child receiving a cancer diagnosis is known to be high, but there is a knowledge gap regarding suicidal behavior among these parents. The aim of this study was to investigate the risk of suicide attempt and death by suicide in relation to having a child with cancer. Methods and findings We performed a binational population-based and sibling-controlled cohort study, including all parents with a child diagnosed with cancer in Denmark (1978 to 2016) or Sweden (1973 to 2014), 10 matched unexposed parents per exposed parent (population comparison), and unaffected full siblings of the exposed parents (sibling comparison). Suicide attempt was identified through the Patient Register and the Psychiatric Central Register in Denmark and the Patient Register in Sweden, whereas death by suicide was identified through the Danish Causes of Death Register and the Swedish Causes of Death Register. In population comparison, we used Cox regression to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) of suicide attempt and death by suicide associated with cancer diagnosis of a child, adjusting for sex, age, country of residence, calendar year, marital status, highest attained educational level, household income, history of cancer, history of psychiatric disorder, and family history of psychiatric disorder. The sibling comparison was performed to assess the role of familial confounding in the studied associations. The population comparison consisted of 106,005 exposed parents and 1,060,050 matched unexposed parents, with a median age of 56 at cohort entry and 46.9% male. During the median follow-up of 7.3 and 7.2 years, we observed 613 (incidence rate [IR], 58.8 per 100,000 person-years) and 5,888 (IR, 57.1 per 100,000 person-years) cases of first-onset suicide attempt among the exposed and unexposed parents, respectively. There was an increased risk of parental suicide attempt during the first years after a child’s cancer diagnosis (HR, 1.15; 95% CI, [1.03, 1.28]; p = 0.01), particularly when the child was 18 or younger at diagnosis (HR, 1.25; 95% CI, [1.08, 1.46]; p = 0.004), when the child was diagnosed with a highly aggressive cancer (HR, 1.60; 95% CI, [1.05, 2.43]; p = 0.03), or when the child died due to cancer (HR, 1.63; 95% CI, [1.29, 2.06]; p < 0.001). The increased risk did not, however, maintain thereafter (HR, 0.86; 95% CI: [0.75, 0.98]; p = 0.03), and there was no altered risk of parental death by suicide any time after the child’s cancer diagnosis. Sibling comparison corroborated these findings. The main limitation of the study is the potential residual confounding by factors not shared between full siblings. Conclusions In this study, we observed an increased risk of parental suicide attempt during the first years after a child’s cancer diagnosis, especially when the child was diagnosed during childhood, or with an aggressive or fatal form of cancer. There was, however, no altered risk of parental death by suicide at any time after a child’s cancer diagnosis. Our findings suggest extended clinical awareness of suicide attempt among parents of children with cancer, especially during the first few years after cancer diagnosis.