Background: Determining the level of physical activity (PA) is an essential part of patient evaluation in axial spondylarthritis (axSpA). Subjective and objective methods are both frequently used methods for evaluating PA. Although subjective methods are cost-effective and easy to use, their accuracy for measuring PA is still questionable. Objective: To investigate the concurrent criterion validity of a self-reported questionnaire (IPAQ-Short Form) when compared to an accelerometer (Actigraph wGT3X-BT) for measuring PA level in patients with axSpA. Design: Cross-sectional design. Methods: Fifty-eight patients with axSpA with a median age of 39.0 (IQR 25/75: 30.0/46.0) years were included in the study. An accelerometer (Actigraph wGT3X-BT) was attached to the waist of patients at their first visits and was removed at their second visits, seven days later. Patients were asked to complete the International Physical Activity Questionnaire Short Form (IPAQ) at their second visits. Results: No significant correlations were determined between IPAQ and accelerometer (p > 0.05), except for the moderate PA (rho: 0.367, p < 0.05), and total PA (rho: 0.330, p < 0.05). It was also observed that IPAQ was underestimating energy expenditure for all types of PA. Conclusion: IPAQ might not be a valid tool for measuring PA level in patients with axSpA. Disease-specific subjective methods for determining the PA should be developed and validated for those patients.