Objectives To compare the construct validity, including discrimination between known groups, of three pain and three morning stiffness (MS) measurement instruments. Methods Patients with radiographic axial spondyloarthritis with 8-year data from the Outcome in Ankylosing Spondylitis International Study cohort were assessed cross-sectionally. Three instruments for pain and three for MS, all self-reported and scored 0–10, were compared. Construct validity was evaluated by testing (1) hypothesis of correlations’ strength and (2) discrimination between known groups using standardised mean differences (SMD) across external constructs. Influence of contextual factors (CFs) on SMDs was investigated. Results Of 85 patients, mean age was 54 (SD 11), mean symptom duration 31 (11) years, 71% males. All six instruments showed a good construct validity by fulfilling >75% of the hypotheses for the strength of correlation. Neck/back/hip pain (Bath Ankylosing Spondylitis Disease Activity Index-Question 2, BASDAI-Q2) and total back pain had higher SMDs compared with back pain at night across all between-group comparisons, with BASDAI-Q2 performing mostly slightly better (eg, SMD for external construct Axial Spondyloarthritis Disease Activity Score (ASDAS; ≥2.1 vs <2.1): 1.87 (BASDAI-Q2) vs 1.56 (total back pain) vs 1.07 (back pain at night)). MS-severity and severity/duration had higher SMDs across all external constructs (with MS-severity slightly better), while MS-duration performed worse (eg, SMD external construct ASDAS: 1.51 (MS-severity) and 1.39 (MS-severity/duration) vs 1.16 (MS-duration)). Influence of CFs on known group discrimination was limited. Conclusions The recommended Assessment of SpondyloArthritis international Society Core Outcome Set (ASAS-COS) pain measurement instrument total back pain BASDAI-Q2 has the best known group discrimination. For MS, the ASAS-COS stiffness measure (MS-severity/duration) performs well although MS-severity even slightly better. Known group discrimination is overall stable across CFs.