To determine if there is an association between sarcopenia, physical function and self-reported fatigue in osteoarthritis (OA) and rheumatoid arthritis (RA). A cross-sectional analysis of measurements from a cohort of 157 participants with OA or RA was performed. The relationship between muscle mass (appendicular muscle index (AMI)), physical function (timed up and go, 30-seconds sit-to-stand test, 40-meter fast-paced walk test and grip-strength) and two fatigue measures (Multidimensional Assessment of Fatigue (MAF) and a fatigue Visual Analogue Scale (VAS)) was explored using hierarchical linear regression or logistic regression with established AMI cut-offs for sarcopenia. There were no significant differences for perceived fatigue-related variables between OA or RA sarcopenic or non-sarcopenic participants. Participants with OA had worse physical function (TUG; P = 0.029, STS; P = 0.004, WS; P = 0.003), but participants with RA had lower grip strength (P<0.001). The RA group had higher CRP (P = 0.006), were more likely to receive glucocorticoids (P<0.001), and experienced worse fatigue (P = 0.050). The hierarchical multiple regression showed that self-reported fatigue (VAS/MAF-distress) had a significant but weak association with AMI in RA. Participants with higher percentage body fat had a significantly stronger association with sarcopenia in both OA and RA. Sarcopenia, when assessed by AMI, does not appear to be strongly associated with self-reported fatigue or physical function in participants with either OA or RA. Higher body fat had a moderately strong association with sarcopenia in this cross-sectional study, suggesting that body composition may be an important factor in the health of patients with longstanding OA or RA.