The current research aimed to investigate the effect of an online training course in content and language-integrated learning (CLIL) on EFL in-service teachers' teaching knowledge, performance and self-efficacy as a venue for their professional development. A pre-post one-group quasi-experimental design was adopted, with twenty EFL in-service teachers as the research participants. Instruments included a semi-structured interview to specify target participants, a teaching knowledge test, a teaching performance observation checklist, and a self-efficacy scale. A professional development Small Private online course (SPOC) in CLIL was designed and uploaded to an online platform (Canvas) to be available for the targeted participants. Results of the study indicated that the teachers demonstrated enhanced teaching competence in both knowledge and performance (cognitive and psychomotor dimensions) according to the statistical analysis of the data obtained through the knowledge test and the teaching performance observation checklist. They also demonstrated enhanced self-efficacy in teaching as indicated by the statistical difference between the pre- and post- administrations of the self-efficacy scale. Such practices are strongly recommended, especially after the COVID-19 pandemic and the constraints of time allowed for those teachers to seek professional development opportunities.