Objective: Morning stiffness (MS) is characteristic of rheumatoid arthritis (RA). Despite its association with functional disability, the extent to which local inflammatory processes contribute to this symptom is unknown. Magnetic resonance imaging (MRI)-detected tenosynovitis of small joints is recognized as an early feature of RA, which is also associated with functional impairments. It has been proposed that tenosynovitis contributes to MS. Therefore, we assessed the relationship between MS and MRI-detected inflammation, in particular tenosynovitis.Method: In total, 286 consecutive patients newly presenting with undifferentiated arthritis and RA underwent contrast-enhanced 1.5 T MRI of (2-5) metacarpophalangeal, wrist, and (1-5) metatarsophalangeal joints. Scans were scored for tenosynovitis according to Haavardsholm, and for synovitis by Rheumatoid Arthritis Magnetic Resonance Imaging Scoring (RAMRIS). MS was dichotomized as ≥ 60 min or not. Associations between MS and tenosynovitis/synovitis were tested with logistic regression, data were categorized (solitary or simultaneous presence of synovitis/tenosynovitis), and the presence of an additive interaction was assessed.Results: MS was present in 40% of patients. Tenosynovitis was more often present in patients with MS than without MS [80% vs 65%, odds ratio (OR) 2.11, 95% confidence interval (1.21;3.69)]. Synovitis was more often present in patients with MS [58% vs 44%, OR 1.79 (1.11;2.91)]. In categorized analyses, concurrent synovitis and tenosynovitis had the largest association [OR 2.43 (1.30;4.54)], in contrast to solitary synovitis [OR 0.85 (0.21;3.47)]. The additive interaction was non-significant. The variance explained in all analyses was small (range 4-5%).Conclusion: Tenosynovitis, combined with synovitis, at small joints is associated with MS and contributes to the pathophysiology of MS.