Objective. Research concerning spinal mobility in axial spondyloarthritis (axSpA) has focused on ankylosing spondylitis (AS), and data on the clinical diagnosis of undifferentiated spondyloarthritis (USpA) are limited. The objective was to study differences in spinal mobility between axSpA subgroups AS and USpA, including gender differences. Methods. A total of 183 patients with axSpA from a rheumatology clinic were included in the study. The earliest recorded spinal mobility measures (cervical rotation/flexion/extension/lateral flexion, tragus-to-wall distance, vital capacity, chest expansion, thoracic flexion, thoracolumbar flexion, lateral spinal flexion, lumbar flexion and intermalleolar distance) were obtained by specialized physiotherapists. Differences between subgroups were analysed using analysis of covariance, controlled for gender and disease duration. Results. In the USpA group (n = 57), the mean [standard deviation (SD)] age was 41.6 (11.4) years, and disease duration was 13 (10.6) years, with 54% men. In the AS group (n = 126), the mean (SD) age was 48.4 (13.5) years, and disease duration 24.6 (13.3) years, with 77% men. Spinal mobility was less restricted in USpA versus AS patients (p = 0.05), with a median (interquartile range) tragus-to-wall distance of 11 (10-12) cm versus 13 (11.3-18.5) cm; thoracolumbar flexion 9 (7-10) cm versus 6.5 (4-9) cm; lateral spinal flexion 29 (25-36) cm versus 21.3 (12-31) cm; lumbar flexion 4.5 (3.5-5.0) cm versus 3.5 (2.0-4.5) cm and intermalleolar distance 113 (102-121) cm versus 101 (86-114) cm. There were no differences between the subgroups in cervical mobility, vital capacity, chest expansion or thoracic flexion, and there were few gender differences, besides anthropometric measures. Conclusion. Patients with USpA and AS had similar cervical and chest mobility, while thoracic and lumbar mobility were more severely restricted in AS. There were few gender differences in either subgroup. Further studies, to understand the full impact of USpA on spinal mobility, are needed. Copyright (C) 2016 John Wiley & Sons, Ltd.