This study examines secondary teachers' efficacy for teaching in a fully online teaching environment during the sudden transition to online teaching due to the COVID-19 pandemic. The study sought to identify how specific variables, teaching experience, professional development (PD) experience, and teaching supports correlate with the self-efficacy perceptions of teachers transitioning to online teaching during a pandemic. More specifically, we examined self-efficacy perceptions of teachers in the domains of student engagement, instructional strategies, classroom management, and computer skills. The instrument used to measure teacher efficacy for online teaching was a 32-item, web-based survey that was given to Ontario secondary teachers in a southern Ontario Catholic district school board. We argue that prior experience with online learning such as additional qualification (AQ) courses or online PD would build greater self-efficacy amongst teachers as they transition to online learning. The results indicate that higher online teaching efficacy scores correlated with having taken online AQ courses and online PD sessions. The highest online teaching efficacy scores correlated with having previously used a board-provided learning management system (LMS) and using virtual technology supports. These indicators are correlated with higher scores of online teaching efficacy but require further investigation as to how they can better provide support for teachers in online learning environments.