We prospectively compared power Doppler ultrasound findings in 25 fingers with rheumatoid arthritis (RA) and 25 fingers with psoriatic arthritis (PsA). Erosive synovitis and tenosynovitis were seen in both groups. Extrasynovial changes were found in 21/24 (84%) fingers with PsA versus none of the fingers with RA. Of the 21 PsA fingers exhibiting extrasynovial changes, 15 (15/25, 60%) also had synovial changes. The extrasynovial changes reflected enthesitis or soft tissue inflammation, with the main patterns being capsular enthesophyte, juxtaarticular periosteal reaction, enthesopathy at the site of deep flexor tendon insertion on the distal phalanx, and subcutaneous soft tissue thickening of the finger pad or entire finger. In four fingers, ultrasonograhy showed pseudotenosynovitis, an underrecognized abnormality characterized by diffuse inflammation of the digital soft tissues. Pseudotenosynovitis may play a pivotal role in dactylitis (sausage digit), which is defined as diffuse uniform swelling of the entire finger. Our findings suggest that inflammation of the fibrous skeleton of the finger may lead to the clinical and radiological features that distinguish PsA from RA of the finger.