The use of self-assessing metacognitive strategies has been shown to enhance English as a foreign language (EFL) learners' achievement in face-to-face classes. However, its impact on EFL learners' achievement in online classes, which have become more prevalent due to the development of new technologies has remained largely unexplored. This study aimed to fill this gap by investigating the effects of online self-assessing metacognitive strategies (OSMLS) accompanied with written languaging on IELTS candidates' listening comprehension (LC) in Iran. A total of 67 IELTS candidates were selected through a convenience sampling method and were homogenized using an IELTS listening test. The participants (n = 44) whose scores were around the mean score were randomly assigned to either a CG (n = 22) or an EC (n = 22). EG received online OSMLS training while the CG did not. Both groups took a pre-test, a post-test, and a delayed post-test of LC. A one-way ANCOVA was used to analyze the collected data. Results indicated that the experimental group outperformed CG on both the post-test and the delayed post-test of LC. The findings suggest that online OSMLS can improve EFL learners' LC skills and have implications for EFL instruction in online settings.