Objective To describe the prevalence of sternoclavicular (SC) joint involvement and the relationship between clinical and ultrasound (US) findings in patients with rheumatoid arthritis (RA). Methods One hundred three consecutive patients with RA and 103 age- and sex-matched healthy individuals were enrolled. Clinical evaluation and blinded US examinations of the SC joint were performed bilaterally in both groups. The presence of gray-scale synovitis, osteophytes, erosions, and intraarticular power Doppler (PD) was recorded. Interobserver agreement was calculated. Results A total of 412 SC joints were evaluated: 206 from patients with RA and 206 from healthy controls. In the RA group, 39 joints (19%) were found to be clinically involved (pain/swelling), in contrast to only 4 (1.9%) in the control group (P = 0.0001). In the RA group, US abnormalities were recorded in 89 SC joints (43%) compared with 36 (17%) in the healthy control group (P = 0.0001), comprising osteophytes in 59 (29%) versus 25 (12%; P = 0.0001), synovitis in 31 (15%) versus 5 (2%; P = 0.0001), erosions in 23 (11%) versus none (P = 0.0001), and intraarticular PD in 5 (2%) versus none (P = 0.03). Furthermore, a correlation between the presence of US synovitis (P < 0.001) and intraarticular PD (P < 0.0001) with a higher Disease Activity Score in 28 joints (DAS28) was found. Conclusion In patients with RA, US detected a higher number of involved SC joints than with clinical assessment. Our results indicate that both gray-scale and PD US findings were more prevalent in patients with RA than in healthy controls. US synovitis and synovial hyperperfusion correlated with the DAS28, suggesting that SC joints actively participate in the systemic inflammatory process of RA.