The rising popularity of online learning and the global pandemic COVID-19 sped up the use of synchronous online teaching modality in HEIs worldwide. But unsatisfactory learning experiences and isolation were prominent and commonly perceived by learners. The study aimed to examine students' sense of belonging in a synchronous online learning environment, explore underlying factors and their interplay, and provide practical suggestions to online program designers and instructors. This study employed an explanatory sequential mixed-method design to answer the research questions. The findings indicated the students engaged in synchronous online learning amid the pandemic did not suffer from severe isolation but insufficient support from empathetic instructors. Instructor, peer, and learning climate and their subordinate factors were the determinants impacting learners' SOB in the learning setting. Based on the combined findings, the study proposed suggestions to improve learners' SOB in synchronous online learning, including setting up a favorable learning climate, establishing common values in the community, stimulating identified regulation and intrinsic motivation of learners, and leveraging technology to improve interaction with learners. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]