BACKGROUND: Rheumatoid arthritis affects physical and mental health; it brings psychological consequences such as dysfunctions in emotion regulation and cognitive-executive functions. The purpose of this study was to investigate and compare the effectiveness of mindfulness-based stress reduction (MBSR) versus cognitive-behavioral therapy (CBT) on emotion regulation, mindfulness, and cognitive-executive functions in patients with rheumatoid arthritis. MATERIALS AND METHODS: This study was a clinical trial with two intervention groups and one control group. The number of 96 patients referred to the rheumatology clinic was selected using random sampling and divided into two intervention groups and a control group. In this study, the Difficulties in Emotion Regulation Scale, Five-Facet Mindfulness Questionnaire, Stroop task, Tower of London task, and Wisconsin Card Sorting Task were used at baseline, posttest, and follow-up. Data were analyzed by SPSS22 using analysis of variance (ANOVA) with repeated measurements. RESULTS: The findings showed that MBSR and CBT have significant effects on mindfulness (P > 001), emotion regulation (P > 001), and cognitive-executive functions (P > 001). Specifically, MBSR led to greater improvements in emotion regulation and mindfulness, while CBT led to greater improvement in cognitive-executive functions. CONCLUSION: MBSR and CBT can be used as effective treatments to improve emotion regulation, mindfulness, and cognitive-executive functions in patients with rheumatoid arthritis.