The rapid development of information and communication technologies (ICT), and the unexpected transition to online teaching due to COVID-19 necessitates that teachers should have the knowledge, competent skills and strategies to integrate digital tools and platforms effectively. Literature suggests however that many teachers do not feel confident enough or lack perceived capability in teaching using advanced technologies in classrooms, and do not have positive self-efficacy beliefs towards their online teaching. Hence, the purpose of this mixed-method study is to investigate teachers' self-efficacy (TSE) in online learning environments amid COVID-19. A total of 150 K-12 teachers from six Arab countries were invited to participate in the study. Quantitative and qualitative data revealed that perceived self-efficacy of online teaching was high. Two main factors, receiving support to design online instruction and receiving professional development in online teaching, significantly predict participants' sense of self-efficacy. Teachers who have previous experience in online teaching scored higher on their self-efficacy than teachers with limited or no experience. Student engagement had the weakest correlation between the four scales with the overall self-efficacy. Parental involvement was discovered through the qualitative analysis to be an emerging factor that could enhance teachers' self-efficacy. Recommendations and limitations are further discussed.