Abstract Background The presence of a mental illness is a known risk factor for suicide mortality and other suicide-related behaviours, including suicidal ideation. We examined prevalence of mental illnesses among adults with and without suicidal ideation in Canada during the pandemic. Data and methods We used pooled data from the 2020, 2021, & 2023 Survey on COVID-19 and Mental Health to estimate the prevalence of moderate to severe symptoms of generalized anxiety disorder, major depressive disorder, and posttraumatic stress disorder among adults who reported suicidal ideation, compared to those who did not report suicidal ideation. We also conducted the analyses across sociodemographic factors and COVID-19 pandemic-related experiences. Results Among adults in Canada who reported suicidal ideation, 83.0% also screened positive for at least one of the three mental illnesses. The prevalence of mental illnesses did not differ significantly across sociodemographic factors or pandemic-related experiences. In contrast, the prevalence of at least one mental illness among adults without suicidal ideation was much lower (21.2%), and this prevalence differed by sociodemographic factors and pandemic-related experiences. Interpretation During the COVID-19 pandemic, most adults in Canada with suicidal ideation had moderate to severe symptoms of mental illness.