Background: Corrected QT (QTc) prolongation is a frequently observed ECG abnormality in patients with rheumatoid arthritis (RA). Objectives: We aimed to investigate the association between disease activity and QTc interval in patients with RA. Methods: Data were obtained from the Kyoto University Rheumatoid Arthritis Management Alliance population-based RA cohort. We used a linear model to compare the association between QTc interval and RA-related parameters, including patient characteristics and disease activity assessed using the visual analog scale, C-reactive protein level, erythrocyte sedimentation rate (ESR), disease activity score 28-joint count using erythrocyte sedimentation rate (DAS28-ESR), simplified disease activity index (SDAI), clinical disease activity index (CDAI), and health assessment questionnaire (HAQ) score. We also constructed multivariate linear regression models to adjust for confounding effects. Results: The mean QTc interval of 340 patients (mean age: 64.7 ± 12.3 years, female: 289 [85%]) with ECG data was 420.0 ± 18.4, and the mean disease activity indices were: DAS28-ESR, 2.7 ± 1.1 points; CDAI, 5.0 ± 5.2 points; SDAI, 5.4 ± 5.7 points; and HAQ, 0.61 ± 0.71 points. Linear correlations were observed between the QTc interval and all parameters for disease activity in the univariate analysis. The three multivariate linear regression models using age, sex, HAQ score, and disease activity indices (CDAI, SDAI, or DAS28-ESR) were significantly associated with the QTc interval (P = 0.0002, 0.0002, 0.0004, respectively). Conclusions: Disease activity is significantly associated with the QTc interval in patients with RA. Attention should be given to ECG abnormalities in patients with RA and progressive disease activity.