It is known that treatment compliance is low and aggression is higher in individuals diagnosed with schizophrenia compared to the normal population. Cognitive behavioural therapy (CBT) is known to reduce relapse and hospitalisation and increase well-being in individuals diagnosed with schizophrenia. However, there are almost no studies on increasing treatment compliance and decreasing aggression. This study was conducted to determine how CBT-based psychoeducation affects medication adherence and aggression in individuals diagnosed with schizophrenia. The study was conducted as a quasi-experimental model with the pre-test-post-test control group with 73 schizophrenic patients (33 experimental, 40 control) between June 2022 and July 2023. Data were collected using the Descriptive Characteristics Form, Morisky Medication Adherence Scale (MMAS) and Buss-Perry Aggression Questionnaire (BPSQ). The schizophrenic patients in the experimental group were given eight sessions of CBT-based psychoeducation, while the schizophrenic patients in the control group were not given any training. The data were analysed using mean, standard deviation, chi-squared test, dependent samples t-test and independent samples t-test. It was determined that the aggression level of the experimental group before the training was 80.51 ± 19.38, and after the CBT-based psychoeducation, it was 73.12 ± 15.28. It was determined that the aggression level of the control group before the training was 84.22 ± 12.13, and after the post-test, it was 85.60 ± 11.72. It was determined that the medication adherence level of the experimental group before the training was 2.75 ± 1.25, and after the CBT-based psychoeducation, it was 3.57 ± 0.67. It was determined that the medication adherence level of the control group before the training was 2.05 ± 1.33, and after the post-test, it was 2.17 ± 1.36. The psychoeducation based on CBT caused a statistically significant difference (p = 0.001) in medication adherence and aggression levels in the experimental group. It was determined that CBT applied to individuals diagnosed with schizophrenia was effective on increasing their medication adherence and reducing their aggression. It is recommended for psychiatric nurses to include CBT-based psychoeducation in their nursing practices in order to increase medication adherence and reduce aggression in individuals with schizophrenia. The findings emphasise that CBT-based psychoeducation increases treatment adherence and significantly decreases the level of aggression in individuals diagnosed with schizophrenia. Psychiatric nurses should include CBT-based psychoeducation in their treatments.