Well-being emerges as a pivotal driver influencing student success, a recurrent theme in existing literature highlighting its integral role in education, particularly in the context of online learning. Consequently, the current study scrutinizes university students' perceptions of their well-being and explores factors affecting their happiness in learning English online. This research consisted of 268 undergraduate students who experienced at least one online English course at a private university in Vietnam and adopted a mixed-method approach, incorporating a questionnaire survey including 15 items of the PERMA (Butler & Kern, 2016) along with 6 items of the Academic Resilience scale (Martin & Marsh, 2006) and individual semi-structured interviews. Descriptive statistics and Pearson's Correlation were utilized for the quantitative data analysis while thematic analysis was applied to the qualitative data. Findings reveal students' perceptions of their well-being when learning online were assessed on average; Academic resilience demonstrated a correlation with student well-being and contributed significantly to their well-being; Students' Grade Point Average (GPA) solely impacted academic achievement construct. Notably, findings uncover negative emotions detrimentally affecting student well-being. Besides these insights, the study's limitations and recommendations for future research were also mentioned. [For the complete proceedings, see ED672804.]