Integrated STEM education holds the promise for students' meaningful science instruction. However, the implementation of integrated STEM-based approaches for science learning might be difficult for teachers due to the lack of expertise and instructional materials. Therefore, this study focused on the preparation and validation of an integrated STEM instructional material (iSTEMim) for genetic instruction among Year 11 science students. The study employed a mixed method design. Five stages of ADDIE which consisted of Analysis, Design, Develop, Implement and Evaluate were adopted to prepare the iSTEMim. Ten (10) science education experts participated in the validation of iSTEMim, and thirty students participated in determining its reliability. A quasi-experimental design was adopted to determine the effects of the prepared iSTEMim. Two secondary schools were randomly assigned to the iSTEMim and traditional group. The two groups were made up of 30 and 32 students respectively. Data were collected using questionnaires and genetic achievement test. The data were analysed using descriptive and t-test statistics. The findings show that iSTEMim has good content validity and its reliability was acceptable. Moreover, the results revealed higher achievement scores for students using the iSTEMim when compared to the traditional method. Consequently, the findings of the study could contribute to the current literature on STEM education research. The iSTEMim could serve as a guide for science educators to implement STEM-based instruction in the classroom.