This action research investigated the effectiveness of reflective portfolio approach in biology instruction among Science and Technology, Engineering, and Mathematics (STEM) learners in the hope of contributing to the corpus of knowledge already available on successful teaching strategies. This study made use of action research approach using both quantitative and qualitative methods of research involving forty (40) Grade 12 STEM (Science, Technology, Engineering, and Mathematics) Senior High School learners. A validated 40-item multiple-choice questionnaire served as the pre-test and post-test for the participants to gauge their understanding of biology topics (Cell Theory, Cell structure, and function) before and after intervention as quantitative data for the study. For qualitative data, a journal was utilized to gather qualitative data from the FGD and interview. The quantitative results revealed that before the instruction using the reflective portfolio, the pre-test scores of the participants were average, while after the implementation of the intervention, the post-test scores of the participants showed a high result. Furthermore, their pre-test and post-test scores in their level of understanding of Biology topics showed a significant difference in the level of understanding of Biology topics. Qualitative data from interviews and focus group discussions also revealed that reflective portfolios improve learners' performance in Biology topics by encouraging critical thinking, promoting deeper understanding, encouraging problem-solving skills, and increasing learners' engagement. The study concludes that reflective portfolios serve as an effective pedagogical tool in biology instruction, fostering a deeper understanding of the subject matter and promoting continuous improvement in the learning practices. [ABSTRACT FROM AUTHOR]