Background: With the growing emphasis on computer science (CS) education in K-12 schools, numerous professional development (PD) programs for in-service CS teachers (CS-PD) have been implemented over the past decades. However, there is a lack of systematic review and meta-analysis to evaluate the effectiveness of these CS-PD programs. To address this gap, this study followed the four "PRISMA" steps to analyze the focused knowledge, teaching practices, and the effect on teaching competence of CS-PD. A systematic review was first conducted based on the 47 empirical articles included in the synthesis and followed by a meta-analysis of 21 of these articles (137 effect sizes). Results: The systematic review (a) identified the 16 sub-themes within CS-related knowledge, (b) summarized three types of teaching practices stressed in CS-PD, and (c) analyzed two levels of methods for evaluating teaching practices. The meta-analysis showed a generally large overall effect size (Hedges' g = 0.623, 95% CI [0.532, 0.714], p < 0.001) for the overall effectiveness of the CS-PD. Conclusions: Therefore, these findings inform future endeavors in implementing an effective CS-PD to promote in-service CS teachers' teaching competence, especially in teaching practices.