Research suggests that teacher self-efficacy shares a relationship with student achievement. This relationship has mostly been examined in face-to-face contexts and it remains to be seen how that relationship translates to online settings. Furthermore, it is unclear whether the methods and instruments used to measure teacher self-efficacy in face-to-face formats are applicable to online settings. The current study explores teacher self-efficacy for K-12 online instructors with data collected using a modified version of a widely used face-to-face Teacher's Self-efficacy Scale (TSES). This new instrument was modified for use in online teaching environments and was used to collect data from K-12 online teachers over 4 time points covering a 6-month span. These data were used to explore the psychometric properties of the new measure. Results suggest that the adapted components of the TSES were valid for use with samples of online teachers with the exception of a new component added to the measure as part of the current study. Continued refinement and use of a modified instrument offers opportunities for further research and will assist online K-12 teachers and administrators and help improve student success.