This study introduces the Teacher Professional Development (TPD) Monitor, an 18-item instrument designed to assess the quality of formal learning opportunities (e.g. courses, workshops), for in-service teachers across all grade levels of general education. By evaluating these opportunities across four critical dimensions—clarity and structure, cognitive activation, collaboration, practical relevance—the TPD Monitor provides a comprehensive framework for assessing the effectiveness of professional development programs. Developed and validated with data from 2314 in-service teachers actively teaching in primary, secondary, special education, and vocational schools, each participating in one of 173 distinct TPD courses, this instrument is supported by multi-level confirmatory factor analysis that confirmed its structure, offering robust evidence of its validity and reliability. Furthermore, measurement invariance was achieved, allowing for meaningful comparisons between subgroups. The TPD Monitor is a valuable and practical tool enabling TPD providers to design effective courses and allowing to monitor and improve the quality of TPD programs. •Introduction of the TPD Monitor - a tool for assessing the quality of teacher PD.•The TPD Monitor covers four dimensions: clarity & structure, cognitive activation, collaboration and practical relevance.•Rigorous psychometric analysis indicates satisfactory results for validity and reliability.•Based on participant ratings, the TPD Monitor offers a quick application for different PD contexts.