States and school districts in the USA have begun to create and implement curriculum to promote elementary students' nascent STEM-related interests and to generate their initial knowledge of careers in those fields. Evaluating the efficacy of such interventions warrants valid and reliable tools, which are not presently available for middle childhood (ages 6-11) aged students in lower elementary school (approximately grades 2 to 4). This research study describes the creation and validation of the "STEM Future-Career Interest Survey (STEM Future-CIS)," a survey informed by extant inventories (i.e., "Student Attitudes toward Science," "Technology," "Engineering," and "Math Survey" and "STEM Career Interest Survey") and grounded in the constructs of interest, self-efficacy beliefs, outcome expectations, and personal goals (i.e., social cognitive career theory or SCCT) to better understand the knowledge and interest in S-T-E-M fields for grades 2-4. From two rounds of student and teacher interviews and pilots punctuated by periods of expert review, 804 students (grades 2-4 in the southeastern U.S.) participated in the "STEM Future-CIS." By employing exploratory and confirmatory factor analyses among four models, results affirmed SCCT constructs as a model for how middle childhood aged students conceive their interest to engage in future career considerations in 25 items and four validated factors of "math/science interest," "engineering interest," "technology interest," and "future self." Sampled students were able to report technology and engineering interests; however, they experienced difficulty in differentiating math and science subject areas and the related future career opportunities in engineering and technology.