Prostate-specific antigen, a widely used biomarker of prostate cancer, lacks specificity and prognostic significance, which often results in over-diagnosis and over-treatment of prostate cancer. In this study, we investigated if seminal plasma proteins could increase specificity of detecting primary prostate cancer and discriminate between low-and high-grade cancers. To select 148 most promising biomarker candidates, we combined transcripts or proteins identified through five independent data mining or experimental approaches: tissue transcriptomics, seminal plasma proteomics, cell secretomics, tissue specificity and androgen regulation. To follow up these candidates, we designed a rigorous biomarker verification and validation pipeline based on multiplex quantitative selected reaction monitoring assays. We verified 77 and validated 19 most promising proteins in seminal plasma of 67 negative biopsy and 152 prostate cancer patients. Validation revealed a prostate-specific, secreted and androgen-regulated protein-glutamine gamma-glutamyltransferase 4 (TGM4), which could predict prostate cancer on biopsy and outperformed age and serum PSA. In the independent validation set re-measured by an in-house ELISA, TGM4 was significantly up-regulated (3.7-fold, P=0.005) and revealed AUC 0.66 for differentiating between negative biopsy and prostate cancer in patients with age ≥50 and blood serum PSA ≥4 ng/mL. Interestingly, none of validated biomarkers could differentiate between low-and high-grade cancers. Very low levels of TGM4 (120 pg/mL) were detected by ELISA in blood serum, but could not differentiate between negative biopsy, prostate cancer or prostate inflammation. Significantly higher levels of TGM4 in blood serum (3.5-fold, P = 0.0004) were found for younger men (<40 y.o.) relative to older men (≥70 y.o.). Our work may represent one of the most comprehensive studies on prostate cancer biomarkers in seminal plasma. Even though moderate performance of TGM4 as a single biomarker will unlikely result in its immediate use in the clinic, TGM4 may be further investigated in the distinct subtypes of prostate cancer and included into emerging multibiomarker panels.