Background: There is no therapeutic competence and adherence scale for grief-focused cognitive behavioural therapy (grief-focused CBT). However, given the growing body of evidence for the efficacy of grief-focused CBT, such a scale is needed both to ensure the internal validity of clinical trials and to facilitate psychotherapy process research. Objective: To develop and undertake a psychometric evaluation of a therapeutic adherence and competence scale for grief-focused CBT. Method: The scale was developed in two steps. (I) Five experts on the treatment of prolonged grief disorder provided feedback on the relevance and appropriateness of the items. The scale was revised to reflect their feedback. The final therapeutic adherence and competence scale for grief (TACs-G) consisted of 15 adherence and 16 competence items. (II) Psychometric evaluation of the TACs-G was based on the rating of 48 randomly selected PG-CBT sessions by two independent raters. The videos were recorded in the context of a randomized controlled trial (RCT; DRKS00012317.) ICC was used to calculate inter-rater reliability and TACs-G stability over time (re-evaluation of 10 sessions after 12 months). Results: The five experts confirmed the relevance and appropriateness of the items. Interrater reliability was found to be high for the total adherence and competence scores (ICC = 0.889 and 0.782, respectively) and moderate to excellent for individual items (ICC = 0.509–1.00). The TACs-G stability over time was found to be strong for both adherence (ICC = 0.970) and competence total scores (ICC = 0.965). Conclusions: The TACs-G for CBT is a reliable instrument that can be used not only to ensure internal validity but is also suited for psychotherapy process studies. Additionally, it provides a valuable database for targeted feedback in training settings. HIGHLIGHTS This is the first study to report on the development and psychometrical evaluation of a grief-focused adherence and competence scale. Although an increasing number of clinical trials do report the efficacy of grief-focused cognitive–behavioural therapy, none of these studies used a standardized adherence and competence scale to control internal validity. In the present study, we introduced a therapeutic adherence and competence scale for grief (TACs-G) that can be applied efficiently across different research settings (e.g. manipulation check, dissemination), and report results of good to excellent psychometric properties. The scale itself could prove useful beyond the research setting as it could possibly serve as a basis for feedback in training settings.