Androgen receptor is the major driver of and testosterone the natural growth factor of prostate cancer (PC). Studies exploring the relationship among circulating testosterone levels, PC aggressiveness, and patient prognosis showed contradictory results. We performed a comprehensive literature search for studies reporting the independent relationship between serum testosterone and prognosis of PC patients. Meta-analyses using random effects models were conducted to estimate pooled hazard ratios (HRs) and 95% confidence intervals (CIs). We identified 25 articles that evaluated the prognostic value of testosterone in early-stage PC (8 studies), in advanced PC either before (4 studies) or during androgen deprivation therapy (ADT) (5 studies), and in castration-resistant PC (8 studies). In early PC, serum testosterone level was not prognostic in terms of overall survival (HR = 1.03; 95% CI, 0.99-1.08; P = .19) and biochemical recurrence (HR = 0.99; 95% CI, 0.87-1.13; P = .93). In advanced PC, higher testosterone levels before ADT were associated with a reduced risk of death (HR = 0.58; 95% CI, 0.45-0.74; P < .0001). During ADT, lower levels were associated with a reduced risk of death (HR = 0.48; 95% CI, 0.28-0.81; P = .006) and progression (HR = 0.59; 95% CI, 0.46-0.77; P < .0001). In castration-resistant PC patients, higher testosterone levels predicted a reduced risk of progression (HR = 0.33; 95% CI, 0.11-0.97; P = .04) but not of death (HR = 0.86; 95% CI, 0.69-1.07; P = .18). The heterogeneity of the included studies is a major limitation of this meta-analysis. The relationship between circulating testosterone and PC prognosis varies in different clinical settings and according to ADT administration.