Objective Oncology nurses are expected to play an important role in psychosocial care for cancer patients. The aim of this study was to examine whether a novel training program aimed at enhancing oncology nurses' ability to assess and manage common psychological problems in cancer patients would improve participants' self‐reported confidence, knowledge, and attitudes regarding care of patients with common psychological problems (trial register: UMIN000008559). Methods Oncology nurses were assigned randomly to either the intervention group (N = 50) or the waiting list control group (N = 46). The intervention group received a 16‐h program, the content of which focused on four psychological issues: normal reactions, clinically significant distress, suicidal thoughts, and delirium. Each session included a role‐play exercise, group work, and didactic lecture regarding assessment and management of each problem. Primary outcomes were changes in self‐reported confidence, knowledge, and attitudes toward the common psychological problems between pre‐intervention and 3 months post‐intervention. Secondary outcomes were job‐related stress and burnout. Intervention acceptability to participants was also assessed. Results In the intervention group, confidence and knowledge but not attitudes were significantly improved relative to the control group. No significant intervention effects were found for job‐ related stress and burnout. A high percentage (98%) of participants considered the program useful in clinical practice. Conclusions This psycho‐oncology training program improved oncology nurses' confidence and knowledge regarding care for patients with psychological problems. Copyright © 2015 John Wiley & Sons, Ltd.