In recent years, saliva samples have attracted attention as specimens, which may be used for cancer diagnosis. Prostate-specific antigen (PSA) is the most useful tumor marker for prostate adenocarcinoma (PA). We examined whether there is an association between saliva PSA and serum PSA in patients with PA using enzyme-linked immunosorbent assay. Human subjects were classified into two groups: a low-serum PSA concentration group (n = 20) (<2.5 ng/mL) and a high-serum PSA concentration group with high risk of recurrence or metastasis (n = 11) (≤2.5 ng/mL). There were significant differences in saliva PSA concentration between these groups (p < 0.05). Saliva PSA concentration correlated very well with serum PSA concentration in the high-serum PSA concentration group (γ = 0.910, p < 0.001) using Spearman's rank test, but no correlation in the low-serum PSA concentration group. This result suggests that saliva PSA is associated with blood PSA in patients with recurrent or metastatic PA and may, therefore, be a useful PA biomarker.