Introduction: Considering the importance of spiritual interventions in the treatment of mental problems, the aim of this research was to investigate the effectiveness of cognitive therapy based on religious beliefs on death anxiety, social adjustment and mental well-being in cancer patients. Materials & Methods: This research was a semi-experimental study with pre/post test and a control group design. 30 cancer pat-ients were selected through convenient sampling method as the sample group in Arezo Charity Institution in Parsabade Mogan city (N=300). The instruments used in this research were Mental Well-being and Social Adjustment and Death Anxiety Scales. The participants in the experimental group were under cognitive therapy based on religious beliefs for 8 sessions each lasting 2 hours. For statistical examination, the analysis of covariance (MANCOVA) was used. Findings: Results showed that there was a significant difference between the mean score of pre-test and post-test of both experimental and control groups.The cog-nitive therapy based on religious beliefs decreased death anxiety scores and also increased the scores of social adjustment and mental well-being in the experimental group rather than the control group. Discussion & Conclusion: Due to the efficacy of cognitive therapy based on religious believes in cancer patients, we can benefit this method as a complementary therapy beside medical treatments and ther-efore use it to improve the mentioned psychological problems in these patients.