Aim: To compare the corticosteroid injection (CSI) with or without thumb spica cast (TSC) for de Quervain tendinitis. Material and methods: The study was conducted in the Department of Orthopaedics, Vardhman Institute of Medical Sciences, Pawapuri, Nalanda, Bihar, India from January 2017 to December 2018. Total 200 patients with de Quervain tenosynovitis and pain on the radial side of the wrist, tenderness at the first dorsal compartment, a positive Finkelstein test, and a pain score greater than 6 were include in this study. The patients were assigned to either TSC + CSI or CSI groups using a random block sequence. All patients in CSI + TSC and CSI groups received 40 mg of methylprednisolone acetate (1 cc) with 1 cc lidocaine 2% by M.M.-K. Using an insulin needle (25 or 27 gauge) in the first dorsal compartment at the point of maximal tenderness. The patients in the CSI + TSC group received a fiberglass TSC as well. Results: The intent-to treat analysis was applied to compare the primary and the secondary outcomes between the CSI + TSC (60 cases) and the CSI (60 cases) groups. Success rate was significantly better in the CSI + TSC group. At the first follow-up visit, the treatment was successful in 58 out of 60 patients in the CSI + TSC group (96.67%) and 48 out of 60 patients (81.67%) in the CSI group (P ¼ .037). the treatment was successful in 58 out of 60 patients in the CSI + TSC group (96.67%) and 48 out of 60 patients in the CSI group (81.67%) (P ¼ 027). The VAS in the CSI + TSC group was 9.7 ± 0.92 before treatment, 0.27± 0.77, 4 weeks after treatment, and 0.44± 0.65 at the final visit (P < .001). The VAS score for the CSI group was 9.1 ±1.5 before treatment, 1.7 ±1 4 weeks after treatment, and 2.3 ±1.7 at the final visit, which were statistically significant (P<.001). The VAS scores changes from the pre treatment visit to the 6-month post-treatment visit were 8.6± 1.7 and 7.3 ±2.3, respectively, suggesting that both treatments were successful in reducing pain. CSI + TSC was, however, significantly more effective in reducing pain (P<.001). Conclusion: The combined technique of corticosteroid injection and thumb spica casting was better than injection alone in the treatment of de Quervain tenosynovitis in terms of treatment success and functional outcomes.