Purpose: Identifying clinically relevant comorbidities and their effect on health-related quality of life (HRQoL) outcomes among men with advanced prostate cancer (APC) can inform patient care and improve outcomes; however, this is poorly understood. The aim of this observational study was to examine the prevalence of comorbidities, and the relationship of comorbidity burden to HRQoL and other patient-reported outcomes (PROs) among men with APC. Methods: Participants were 192 men (average age 68.8) with APC (stage III or IV) who completed a psychosocial battery including measures of sociodemographic factors, HRQoL and other PROs, and the Charlson Comorbidity Index (CCI). Hierarchical multiple regression analysis was used to examine the relationships between CCI, HRQOL, and PROs. Results: The vast majority (82%) of participants had at least one comorbidity, with the most common being: hypertension (59%), connective tissue disease or arthritis (31%), diabetes (24%), and problems with kidneys, vision, or another organ (24%). After controlling for covariates, regressions showed that a higher CCI score was significantly associated with worse HRQoL ( p < 0.001), lower levels of positive affect ( p < 0.05), and higher levels of depression ( p < 0.05), fatigue ( p < 0.001), pain ( p < 0.01), stress ( p < 0.01), and cancer-specific distress ( p < 0.05). Conclusions: Comorbidities were common among men with APC, and a greater CCI score was associated with detriments in several domains of HRQoL and other PROs. Our findings show the need to address comorbidities in the presence of a cancer diagnosis and subsequent treatment.