Background <p>Suicide rates are higher in prison environments than those in the general population. Therefore, preventive activities are crucial. We aimed to evaluate the effectiveness of a psychoeducational group intervention in decreasing the number of suicidal behaviors in prisons.</p> Methods <p>We conducted a randomized controlled trial in nine prisons in Catalonia. Main outcome: total number of suicidal behaviors. Other variables: suicidal ideations, depressive and anxiety symptoms, and quality of life. The intervention group carried out 17 psychoeducation sessions and the control group received information.</p> Results <p>171 individuals participated in the study, mean age of 34.9 years, 84.8% males. The intervention group (<i>n</i> = 85) reduced anxiety and depression and increased quality of life post-intervention (<i>p</i> &lt; 0.05), with no changes in the control group (<i>n</i> = 86). Both groups reduced all suicide items except for the number of interrupted/aborted attempts and the percentage of people with non-suicidal self-injurious behavior, which improved only in the intervention group. In both groups, the absolute number of suicide attempts decreased (<i>p</i> &gt; 0.05).</p> Conclusions <p>The psychoeducational intervention had an impact on suicidal ideation observed in both groups, with a non-significant reduction in the number of suicide attempts. Furthermore, the intervention improved levels of anxiety, depression, and quality of life.</p> Trial registration <p>This trial was registered at ClinicalTrials.gov on January 19, 2022 (NCT05195554).</p>