Purpose: This article describes the development and initial validation of the Speech-Language Pathologist (SLP) Involvement in Multi-Tiered System of Supports (MTSS) Questionnaire. It was developed to measure the extent to which SLPs are involved in MTSS at their school site(s). Method: A total of 567 SLPs practicing in the United States responded to up to 39 Likert-type items meant to reflect six domains: scope of practice, professional development, leadership, consulting/collaborating, assessment and analysis, and intervention. Measurement quality was evaluated in terms of score reliability and validity. An exploratory factor analysis was conducted to evaluate the internal structure of the questionnaire responses. A three-factor model with the following dimensions of Carrying out Roles and Responsibilities, Leading, and Planning and Providing Interventions was supported. Loadings for retained factors ranged from .35 to .87. Internal consistency estimates ranged from 0.87 to 0.92. Descriptive statistics summarized the overall involvement of SLPs in MTSS, and responses to questions to assess the feasibility and acceptability of this questionnaire were analyzed. Results: Questionnaire responses indicated that SLPs infrequently engage in MTSS activities. The item with the highest mean was related to SLPs collaborating with teachers to help them address students' speech and language disorders in their classrooms. Most of the SLPs who took the questionnaire found it easy to complete, but only some found the information to be useful. Conclusions: The infrequent involvement of SLPs in MTSS indicates a need to disseminate information on the potentially valuable roles SLPs can play in MTSS implementation. This tool may be used by SLPs to better understand contributions they can make within an MTSS framework and self-reflect on their current levels of involvement.