The research was devoted to substantiating, developing, and validating the diagnostic tool for assessing the teacher’s professional competence development. The self-assessment was focused on five professional development components (Motivation and Value, Cognitive-information, Behaviour, and Activity, Personal Reflection, Emotion, and Volition) and was realized according to the Motivational, Cognitive, Activity, Reflection, and Volition criteria with the corresponding indicators. A small-scale experiment was provided to validate the diagnostic tool. It included 52 Ukrainian agricultural and technical college teachers who passed the self-assessment in the first stage of the experiment and the expert assessment in the second stage. Statistical processing of the data showed no significant difference between the expert evaluation and self-assessment results. That proved the validity of the developed diagnostic tool. A full-scale experiment in 2023 included 882 engineering teachers from Ukrainian technical colleges. The results demonstrated high levels of motivational and value, behavior and activity, personal reflection, emotion and volition components, and a lower level of cognitive-informational components of teachers’ professional development. That demonstrated the need for engineering teachers to have advanced knowledge in the educational process organization, innovative methods and forms of teaching engineering students, and designing up-to-date learning content.