Individualized metacognitive training (MCT+) is a novel psychotherapy that has been designed to specifically target delusional beliefs in people with psychosis. It works by developing an awareness of the implausible content of delusional beliefs, while also targeting the cognitive biases that contribute to their formation and maintenance. It was expected that MCT+ would lead to significantly greater reductions in delusional severity compared to a cognitive remediation (CR) active control condition. A total of 54 patients with a schizophrenia spectrum disorder and active delusions were randomized into four 2-hourly sessions of MCT+ (n = 27) or CR (n = 27). All participants completed posttreatment assessment, and only 2 participants did not complete 6-month follow-up assessment, resulting in MCT+ (n = 26) and CR (n = 26) for final analysis. The primary outcome measures of delusional and positive symptom severity were assessed rater-blind; secondary outcome assessment was non-blinded and included clinical and cognitive insight, the jumping to conclusions (JTC) bias, and cognitive functioning. Participants in the MCT+ condition showed significant reductions in delusional and overall positive symptom severity (large effect) and improved clinical insight (moderate effect) relative to CR controls. In contrast, CR controls showed moderate improvement in problem-solving ability relative to MCT+, but no other cognitive domain. Importantly, these findings were maintained at 6-month follow-up. The study adds further efficacy to the MCT program, and suggests that even brief psychotherapy can help to ameliorate the symptoms of psychosis.