Arthritis patients may show little motivation for physical activity (PA), resulting in a sedentary lifestyle. The primary objective of the study was to investigate whether motivation for PA and fulfillment of PA recommendations were associated with cardiorespiratory fitness in patients with RA. The exploratory objective was to study whether university students could be used as controls for RA patients in future studies of PA motivation. Peak oxygen uptake (VO) was measured in 93 RA patients. The patients and 354 students filled in the Behavioral Regulation in Exercise Questionnaire-2 (BREQ-2). Data were analyzed using structural equation modeling with adjustment for age and sex. The BREQ-2 scores were also compiled to an overall motivational style "Relative Autonomy Index" as previously published. Mean VO for the RA patients was 32.2 (SD: 9.6) mL × min × kg. Only 29 patients (31%) fulfilled the current recommendations for PA. BREQ-2 scores were associated with measured VO (standardized coefficient 0.33, p < 0.001). Whether a person fulfilled the current recommendations for PA was a significant mediator of this effect (standardized coefficients: mediated effect; 0.22, p = 0.001, remaining direct effect; 0.11, p = 0.18). The Relative Autonomy Index also significantly predicted measured VO (standardized coefficient 0.30, p < 0.001). The underlying BREQ-2 factor structure was significantly different between RA patients and university students, and comparison of scores would not be adequate. Motivation for PA was significantly associated with measured VO in RA patients. The effect was mediated by whether the patient fulfilled the current recommendations for PA. Addressing and stimulating motivation is important when intervening to increase PA and cardiovascular fitness in RA patients.