The hallmark clinical sign of rheumatoid arthritis is morning stiffness lasting more than 1 hour. This prolonged stiffness reflects the chronic inflammatory process characteristic of RA, resulting in synovial inflammation and joint swelling. Unlike osteoarthritis, where stiffness typically resolves within 30 minutes, RA's persistent stiffness improves gradually with activity. This duration of morning stiffness is a key diagnostic feature and helps differentiate RA from other joint disorders.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.