Standardised patients (SPs) are increasingly utilised in health sciences education to assist students in the development of clinical competencies, including interviewing skills. This study investigated the development and validation of a rating scale for formative assessment of speech pathology students in an interview with an SP. Participants in this study were 76 undergraduate speech pathology students and 10 clinical educators who participated in a simulated clinic module. As part of the module, pairs of students interviewed an SP portraying a parent of a child with speech delay. The Standardised Patient Interview Rating Scale (SPIRS) was developed to assess students' foundation clinical competencies of communication, interviewing and professional practice skills. Students' interviews were videotaped, rated individually on the SPIRS by the clinical educator, and later re-rated by an expert rater. Data were analysed to determine the content validity, internal consistency, and inter-rater reliability of the tool. In addition, descriptive statistics were used to report student performance levels. Results indicated that the SPIRS had good content validity and internal consistency but that there may be some redundancy in individual items. An acceptable level of inter-rater reliability was achieved. Students generally scored highly, with non-verbal communication being the easiest and professional practice the most difficult skill to demonstrate. The SPIRS was found to be an appropriate tool for formative assessment of students in this simulated clinic module. Recommendations for improving its reliability were made. Further research is required to investigate use of the SPIRS as an assessment tool in other contexts utilising standardised patients.