This study determined the effect of online teaching in Physics using the Just-in-Time Teaching (JiTT) strategy on the academic achievement and conceptual understanding of Grade 9 students. One intact class was subjected to a single-group pretest/posttest pre-experimental research design. Purposive sampling was applied, and selected 48 Grade 9 students for this study. The data gathered were interpreted quantitatively from the validated physics achievement test (PAT) and from the adopted energy-momentum concept test (EMCT), while, the learning experiences survey responses were interpreted (...) thematically. This study was conducted for a period of eight (8) weeks during the 4th quarter of the school year 2020-2021 at Junior High School in District II. The study's findings using descriptive and inferential statistics revealed a significant difference in the students' physics achievement mean scores and a significant difference in the students’ energy-momentum mean scores before and after their exposure to online teaching in physics using the Just-in-Time Teaching (JiTT) strategy. The class average normalized gain (g) of the 48 respondents on the adopted Energy-Momentum Concept Test (EMCT) resulted in statistically “high” normalized gain (g) in their conceptual understanding before and after their exposure to online teaching in physics using the Just-in-Time Teaching (JiTT) strategy. Furthermore, the learning experiences survey responses revealed positive results and improvement on the students’ learning experiences in JiTT’s warm-up exercises. Hence, it was concluded that the online teaching in physics using the Just-in-Time Teaching (JiTT) as a teaching-learning strategy might significantly improve the students' academic achievement and conceptual understanding in grade 9 physics. (shrink)