Objective: To compare dimensionality, item-level characteristics, scale-level reliability, and construct validity of PROMIS Physical Function short forms (PROMIS-PF) and 24-item Roland Morris Disability Questionnaire (RMDQ-24) in patients with chronic low back pain (LBP). Design: Cross-sectional study. Setting: Secondary care center for rehabilitation and rheumatology. Participants: Patients with nonspecific LBP ≥3 months (N=768). Mean age was 49±13 years, 77% were female, and 54% displayed pain for more than 5 years. Interventions: Not applicable. Main Outcome Measures: Dutch versions of the 4-, 6-, 8-, 10-, and 20-item PROMIS-PF and of the RMDQ-24. Results: PROMIS-PF-6, PROMIS-PF-8, and RMDQ-24 exhibited sufficient unidimensionality (confirmatory factor analysis: comparative fit index>0.950, Tucker-Lewis index>0.950, root means square error of approximation<0.060), whereas the other instruments did not. All instruments were free of local dependence except PROMIS-PF-20 with 4 item pairs with clear residual correlations. Mokken scale analysis found 1 nonmonotone item for PROMIS-PF-20 and 8 for RMDQ-24 (ie, the probability of endorsing these items was not increasing with increasing level on the underlying construct). PROMIS-PF-20 displayed 2 misfitting items (S-χ2 P value>.001). Two-parameter item response theory models found 2 items with low discrimination for RMDQ-24. All other instruments had adequate fit statistics and item parameters. PROMIS-PF-20 displayed the best scale-level reliability. Construct validity was sufficient for all instruments as all hypotheses on expected correlations with other instruments and differences between relevant subgroups were met. Conclusions: PROMIS-PF-6, PROMIS-PF-8, and RMDQ-24 exhibited better unidimensionality, whereas PROMIS-PF-4, PROMIS-PF-6, PROMIS-PF-8, and PROMIS-PF-10 showed superior item-level characteristics. PROMIS-PF-20 was the instrument with the best scale-level reliability. This study warrants assessment of other measurement properties of PROMIS-PF short forms in comparison with disease-specific physical functioning instruments in LBP.