Aims: To develop the Jaw Functional Limitation Scale (JFLS), comprising 3 constructs and a global scale, based on a preliminary instrument, and to investigate content validity of the overall functional limitation construct, reliability, and generalizability. A temporomandibular disorders (TMD) patient group, compared to other diagnostic groups, was hypothesized to report further limitation in each of the 3 new proposed constructs. Methods: One hundred thirty-two consecutive patients from 5 diagnostic groups (TMD, primary Sjögren syndrome, burning mouth syndrome, skeletal malocclusion, and healthy controls) participated in a known-groups validity design. Fifty-two jaw functional limitation items were identified by an expert panel for content validity. Rasch methodology was used for item reduction and assessment of model fit. The instrument was retested 1 to 2 weeks later. Results: Three constructs (mastication, vertical jaw mobility, and emotional and verbal expression) comprising a total of 20 items were identified along with a global scale (the JFLS-20), and each exhibited excellent psychometric properties with respect to modeled variance, item fit, reliability, and internal consistency. The psychometric properties of each construct remained satisfactory when analyzed separately among the 5 diagnostic groups. Temporal stability was satisfactory. A shorter 8-item form (JFLS-8) also proved useful for assessing global functional jaw limitation. Conclusion: The JFLS-20 is an organ-specific instrument comprising 3 constructs for assessing functional status of the masticatory system; the 3 scales exhibit properties that are ideal for both research and patient evaluation in patient groups with a range of functional limitations of the jaw. The JFLS-8 emerged as a short form for measuring global functional limitation of the jaw. Keywords: functional limitation, masticatory system, oral health-related quality of life, reliability, validity