As the First-In-Human study of in situ gene therapy using an adenovirus vector carrying the human REIC (reduced expression in immortalized cell)/Dkk-3 gene (Ad-REIC), we conducted neoadjuvant intraprostatic injections in patients with high-risk localized prostate cancer undergoing radical prostatectomy (RP). Patients with recurrence probability of 35% or more within 5 years following RP, as calculated by Kattan's nomogram, were enrolled. Patients received two ultrasound-guided intratumoral injections at 2-week intervals, followed by RP 6 weeks after the second injection. After confirming the safety of the therapeutic interventions with initially planned three escalating doses of 1.0 × 1010, 1.0 × 1011 and 1.0 × 1012 viral particles (vp) in 1.0–1.2 ml (n = 3, 3 and 6), an additional higher dose of 3.0 × 1012 vp in 3.6 ml (n = 6) was further studied. All four DLs including the additional dose level-4 (DL-4) were feasible with no adverse events, except for grade 1 or 2 transient fever. Laboratory toxicities were grade 1 or 2 elevated aspartate transaminase/alanine transaminase (n = 4). Regarding antitumor activities, cytopathic effects (tumor degeneration with cytolysis and pyknosis) and remarkable tumor-infiltrating lymphocytes in the targeted tumor areas were detected in a clear dose-dependent manner. Consequently, biochemical recurrence-free survival in DL-4 was significantly more favorable than in patient groups DL-1+2+3.