Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease that causes synovitis in the peripheral joints as well as extra-articular symptoms. With a good response to conventional drugs, treatment for this disease remains unsatisfactory. Vitamin D deficiency is now widely recognized as a problem in RA patients. Some recent trials have attempted to analyze the effect of vitamin D3 supplementation on pain intensity and disease activity in patients, and the majority of them have found significant improvement. The goal of this trial was to see if supplementing vitamin D3 with conventional synthetic DMARDs improved the intensity of pain and disease activity in RA patients. This study was a randomized, double-blind, placebo-controlled trial and was conducted in the Department of Pharmacology, BSMMU in collaboration with the Rheumatology Rehabilitation Clinic of the Department of Physical Medicine and Rehabilitation, BSMMU. Patients were assessed using a validated version of the visual analog scale (VAS) and the DAS-28 CRP scale. A total of 58 RA patients were selected based on inclusion and exclusion criteria, and baseline CRP levels were measured serum preserved for vitamin D3 estimation. A physiatrist at the Physical Medicine and Rehabilitation department's Rheumatology Rehabilitation Clinic performed the clinical diagnosis of RA patients. Following the completion of the necessary formalities, including the patients' informed consent, the patient has given a Visual Analog Scale (VAS) questionnaire to assess pain improvement and a disease activity score -28 C-Reactive Protein (DAS-28 CRP) scale to assess disease activity. The patients were split into two arms at random: intervention and control. Patients in the intervention arm received csDMARDs plus vitamin D3 (40,000IU) for 8 weeks. Weekly oral vitamin D3 (40,000IU) was given to the intervention arm. On the other hand, The placebo arm got csDMARD in the form of one oral placebo capsule once a week for the same duration of time. After 8 weeks of therapeutic intervention, blood samples were taken to determine serum 25 hydroxyvitamin D and CRP levels. The baseline and after 8 weeks serum 25 hydroxyvitamin D levels were estimated at the same time. The patient's compliance sheet, as well as the phone and pill count, were used to verify that medicine intake was regular. Microsoft Office Excel 2007 was used to perform the statistical analysis. A chi-squared test was used to analyze the relationship between the intervention and placebo arms. An unpaired t-test was used to compare the scores of the two arms. A paired t-test was employed to compare the score before and after the intervention. A total of 58 people were enrolled in this study over six months. Following decoding, it was established that twenty-nine (30) patients would receive vitamin D3 and twenty-three (28) would receive a placebo. Among them, fifty-two (52) patients met all criteria to be eligible for analysis. There was a substantial difference in improving disease activity and pain severity between the two arms (p-value- 0.00). All of the RA patients were vitamin D deficient. Vitamin D3 supplementation for 8 weeks resulted in a significantly higher level than the placebo arm (p- 0.01), as well as a significant increase from the baseline level (p- 0.04). After 8 weeks of vitamin D3 administration, CRP levels were significantly reduced (p-0.00). There was no correlation between serum 25-hydroxyvitamin D levels and disease activity. Vitamin D3 significantly reduced the intensity of pain and disease activity in conventional synthetic Disease-Modifying Antirheumatic Drugs (csDMARD) treated Rheumatoid Arthritis patients.