The past two decades have shown a rising global trend to offer online K-12 STEM learning, necessitating teachers to have the knowledge and skills to navigate online teaching contexts. However, related professional development and online STEM best teaching practices remain to be fully articulated. This issue was exacerbated following the COVID-19 pandemic outbreak that pushed most teachers into emergency remote teaching (ERT) roles without preparation. As such, this study explores secondary master STEM teachers' (e.g., > 8 years of STEM teaching experience) transitions to ERT, classifying and categorizing what tensions they encountered in the process. Survey methods and open coding were used to collect and analyze data emphasizing teachers' perceived challenges in shifting to ERT. Findings suggest that while participants had considerable STEM teaching experience, they encountered converging conceptual, pedagogical, cultural, and political tensions connected within ERT contexts. Results offer tangible starting points for supporting teachers in transitioning to online STEM environments.