The Science Teaching Efficacy Belief Instrument A (STEBI-A; Riggs & Enochs, 1990 in Science Education, 74(6), 625-637) has been the dominant measurement tool of in-service science teacher self-efficacy and outcome expectancy for nearly 30 years. However, concerns about certain aspects of the STEBI-A have arisen, including the wording, validity, reliability, and dimensionality. In the present study, we revised the STEBI-A by addressing many concerns research has identified, and developed a new instrument called the T-STEM Science Scale. The T-STEM Science Scale was reviewed by expert panels and piloted first before it was administered to 727 elementary and secondary science teachers. The combination of classical test theory (CTT) and item response theory (IRT) approaches were used to validate the instrument. Multidimensional Rasch analysis and confirmatory factor analysis were run.Based on the results, the negatively worded items were found to be problematic and thus removed from the instrument. We also found that the three-dimensional model fit our data the best, in line with our theoretical conceptualization. Based on the literature review and analysis, although the personal science teaching efficacy beliefs (PTSEB) construct remained intact, the original outcome expectancy construct was renamed science teacher responsibility for learning outcomes beliefs (STRLOB) and was divided into two dimensions, above- and below-average student interest or performance. The T-STEM Science Scale had satisfactory reliability values as well.Through the development and validation of the T-STEM Science Scale, we have addressed some critical concerns emergent from prior research concerning the STEBI-A. Psychometrically, the refinement of the wording, item removal, and the separation into three constructs have resulted in better reliability values compared to STEBI-A. While two distinct theoretical foundations are now used to explain the constructs of the new T-STEM instrument, prior literature and our empirical results note the important interrelationship of these constructs. The preservation of these constructs preserves a bridge, though imperfect, to the large body of legacy research using the STEBI-A.