Students' strongly positive STEM interest and identity predict their future study and career choices in a STEM field. STEM education studies addressing multiple disciplines together are insufficient, as they have produced mixed findings and inadequate direction for advancing integrated STEM education. Self-determination theory (SDT) provides an understanding of motivational processes that influence the development of STEM interest and identity. This study investigated the effectiveness of a set of proposed teacher needs-supportive strategies on student STEM interest and identity development during a proposed 12-week SDT-based STEM program. Three hundred forty-two ninth grade students were randomly assigned to SDT and non-SDT groups during the program. The results support the application of SDT in integrated STEM learning and explain how supporting student needs affects their STEM interest and identity, which is crucial in interdisciplinary learning and the development of adolescent interest and identity in K-12. Moreover, the results contribute to SDT by adding a new dimension--integrated STEM interest and identity--and presenting more evidence on how the teacher's needs-supportive strategies foster this dimension. These results have practical implications for advancing integrated STEM education in addition to new opportunities for using fewer resources to effectively foster student interest and identity in compulsory education.