Objective Negative intrusive thoughts about one's prostate cancer have been associated with depressive mood and impaired quality of life among prostate cancer patients. However, little is known about possible predictors for negative intrusive thoughts among this group. We aimed to identify health‐ and care‐related predictors for such thoughts among a population of men newly diagnosed with prostate cancer and undergoing radical prostatectomy. Methods In the LAPPRO‐trial, 3154 men (80%) answered study‐specific questionnaires at admission and 3 months after surgery. Questions concerned socio‐demographics, health, uncertainty, preparedness for symptoms, and the outcome—negative intrusive thoughts. Associations between variables were analyzed by log‐binominal and multivariable approach. Results The strongest predictor of negative intrusive thoughts at admission to surgery was uncertainty of cure, followed by binge drinking, poor physical health, antidepressant medication, not being prepared for urinary symptoms, age under 55, and physical pain. Reporting it not probable to obtain urinary symptoms after surgery lowered the odds. Negative intrusive thoughts before surgery were the strongest predictor for such thoughts 3 months later followed by uncertainty of cure, physical pain, younger age, living alone, and poor self‐reported physical health. Conclusions Our findings showed an association of preoperative uncertainty of cure as well as low preparedness for well‐known surgery‐induced symptoms with higher occurrence of negative intrusive thoughts about prostate cancer. Future studies should examine if interventions designed to have healthcare professionals inform patients about their upcoming prostatectomy reduce patients' negative intrusive thoughts and thereby, improve their psychological well‐being.