Interprofessional collaboration is recommended in health systems everywhere, but research on its measurement is needed. This psychometric study assessed the construct validity, internal consistency, and test-retest reliability of the Intensity of Interprofessional Practices Questionnaire for Private-Sector Physiotherapists (IIPQ-PT). A random sample of 303 physiotherapists (PTs) completed the 12-item questionnaire, and a subsample of 103 completed it a second time 2 weeks later. Exploratory factor analyses revealed a one-factor solution for the instrument. IIPQ-PT scores showed respectively fair and moderate relationships with the percentage of low back pain clients for whom PTs reported interactions with other professionals (0.30; 95% CI 0.19-0.40; p<0.001) and perceived degree of interactions (0.58; 95% CI 0.50-0.65; p<0.001). The IIPQ-PT had high internal consistency (Cronbach's α=0.86; 95% CI 0.83-0.88) and good test-retest reliability (ICC=0.69; 95% CI 0.57-0.78). The minimum detectable difference for a 95% CI was 2.52. Although further study of this instrument is warranted, our results are promising for its future use. The IIPQ-PT may prove useful to evaluate the effectiveness of interventions aiming to improve interprofessional practices and to measure the effects of such practices on service users, providers, and the health system.