Importance: Solid-organ transplant recipients (SOTRs) are at greater risk of developing some cancers than the general population; however, because they are also at increased risk of mortality from noncancer causes, the effect of transplantation on cancer mortality is unclear. Objective: To describe cancer mortality in SOTRs and to assess whether SOTRs are at increased risk of cancer mortality compared with the general population. Design, Setting, and Participants: Population-based cohort study of patients who underwent solid-organ transplantation in Ontario, Canada, between 1991 and 2010 with 85 557 person-years of follow-up through December 31, 2011. Solid-organ transplantation was identified using the national transplant register and linked to the provincial cancer registry and administrative databases. The analysis was conducted between November 2013 and February 2015. Exposure: Solid-organ transplantation. Main Outcomes and Measures: Cancer mortality for SOTRswas compared with that of the general population using standardized mortality ratios (SMRs). Mortality and cause of death were ascertained by record linkage between the Canadian Organ Replacement Register, the Ontario Cancer Registry, and the Office of the Registrar General of Ontario death database. Results: A total of 11 061 SOTRs were identified, including 6516 kidney, 2606 liver, 929 heart, and 705 lung transplantations. Recipients had a median (interquartile range) age of 49 (37-58) years, and 4004 (36.2%) were women. Of 3068 deaths, 603 (20%) were cancer related. Cancer mortality in SOTRs was significantly elevated compared with the Ontario population (SMR, 2.84 [95%CI, 2.61-3.07]). The risk remained elevated when patients with pretransplant malignant neoplasms (n = 1124) were excluded (SMR, 1.93 [95%CI, 1.75-2.13]). The increased risk was observed irrespective of transplanted organ. The SMR for cancer death after solid-organ transplantation was higher in children (SMR, 84.61 [95%CI, 52.00-128.40]) and lower in patients older than 60 years (SMR, 1.88 [95%CI, 1.62-2.18]) but remained elevated compared with the general population at all ages. Conclusions and Relevance: Cancer death rate in SOTRs was increased compared with that expected in the general population; cancer was the second leading cause of death in these patients. Advances in prevention, clinical surveillance, and cancer treatment modalities for SOTRs are needed to reduce the burden of cancer mortality in this population.