Background: Carpal tunnel syndrome (CTS) can affect 4-50% of rheumatoid arthritis (RA) patients. Pain and paresthesia in the first three digits are the commonest symptoms of CTS and can progress to hand weakness in the late stage. Objective: This study aimed to determine the most prevalent anatomical cause of CTS in non-clinically deformed RA. Patients and methods: This case-control study included convenience sample of 100 female RA patients with CTS and clinically non-deformed wrists who were recruited from the Outpatient Clinics, Faculty of Medicine, Ain Shams University Hospitals over a period of three months. Results: The patients' average age was 44.68 ± 6.97 years. Musculoskeletal ultrasound showed that 42% of the patients had tenosynovitis, 30% of the patients had thickened flexor retinaculum, 22% of the patients had increased power Doppler (PD) signal, and 6% of the patients had bone erosions. There was a significant association between tenosynovitis and age, duration of RA, and moderate CTS, compared to patients with thickened flexor retinaculum, increased PD signal, and bone erosions. Conclusion:  There was an inflammatory pattern present in the ultrasound spectrum of CTS in RA patients, such as tenosynovitis and/or synovitis. Tenosynovitis was a key factor in the onset of CTS in RA.