Self-efficacy has emerged as a popular construct in second language research, especially in the frontline and practitioner-researcher spaces. A troubling trend in the relevant literature is that self-efficacy is often measured in a general or global manner. Such research ignores the fact that self-efficacy is a smaller context-driven construct that should be measured within a specific task or activity where time, place, and purpose domains are considered in the creation of the measurement. Task-based language teaching researchers have also largely neglected the affective factors that may influence task participation, including self-efficacy, despite its potential application to understanding task performance. In this report, we present an instrument specifically developed to measure English as a foreign language students' self-efficacy beliefs when performing a dialogic, synchronous, quasi-formal group discussion task. The instrument's underlying psychometric properties were assessed (N = 130; multisite sample from Japanese universities) and evidence suggested that it could measure a unidimensional construct with high reliability. The aggregate scale constructed from the instrument's items also displayed a central tendency and normal unimodal distribution. This was a positive finding and suggested that the instrument could be useful in producing a self-efficacy measurement for use in the testing designs preferred by second language researchers. The potential applications of this instrument are discussed while highlighting how this report acts as an illustration for investigators to use when researching self-efficacy.