Outbreak of COVID-19, online examination, and e-proctoring have caused more exam anxiety and affected exam performance among students' studying in International Baccalaureate (IB) Diploma Programme (DP). Therefore, the present research aimed to find effect of online test anxiety on academic performance of IBDP students in the subjects related to science, technology, engineering, and mathematics (STEM). Study employed quantitative descriptive survey research design and administered survey questionnaire to 200 IB DP students who took online test during COVID-19 through convenient sampling technique. Sample included both first and final year DP students with due representation to boys and girls. Results of the study revealed a moderate negative correlation between online test anxiety and academic performance of IBDP students in STEM subjects. Regression analysis explained 14.1% variation in the STEM subject performance because of online exam anxiety under e-proctored condition. There exists a statistically significant difference between first and final year students' online exam anxiety and STEM subject average grades. Future research may focus on conducting comfortable online examination methods with no additional exam anxieties.