OBJECTIVE: Being physically active postdiagnosis has been associated with lower rates of prostate cancer progression and mortality, but studies investigating postdiagnostic time spent sitting are lacking. We aim to study the association between leisure time sitting after a prostate cancer diagnosis and overall and prostate cancer-specific mortality. METHODS: Data from 4595 men in Sweden, diagnosed with localized prostate cancer between 1997-2002 and followed-up until the end of 2012, were analyzed. Time spent sitting during leisure time postdiagnosis was categorized into <2, 2-3, 3-4, and >4 h/day. Multivariable-adjusted Cox proportional hazards models were used to estimate hazard ratios (HRs) with 95% confidence intervals (CI) of postdiagnosis leisure time sitting and a joint variable of sitting time and exercise, and time to overall or prostate cancer-specific death. RESULTS: The results showed no significant associations between postdiagnostic leisure time sitting and overall or prostate cancer-specific mortality rates. When the joint effect of both sitting and exercise time was considered, borderline significantly lower mortality rates for overall and prostate cancer-specific mortality were seen among participants that sat the least and exercised the most compared to the reference category with participants sitting the most and exercising least (HR: 0.75; 95% CI: 0.56-1.00 and HR: 0.61; 95% CI: 0.36-1.05, respectively). CONCLUSIONS: No significant association between leisure time sitting and mortality rates among men diagnosed with localized prostate cancer was seen. This study does not support an association between leisure time sitting per se; however, being physically active may have beneficial effects on survival among men diagnosed with localized prostate cancer.