Purpose: A lack of research has reported how playful gamification is applied to adult learners as an idea of andragogical instruction. Thus, this study aims to identify how the concept of gamification was used for adult learners in an online class during the COVID-19 pandemic and its impact on learning performance and motivation with the guidance of Knowles' andragogical principle. Design/methodology/approach: The study applied an explanatory sequential mixed method in collecting the data. Assessments' scores during the experimental research and questionnaires were used as the quantitative data. For the qualitative data, personal semi-structured interviews were employed. Findings: The findings indicate that gamification raises student enthusiasm and interest and improves learning outcomes. Students who previously lacked attentiveness to the online class are now waiting for game quiz activities during the class meeting. Furthermore, the experimental groups reported statistically improved assessments compared to their counterparts. Indeed, some recommended other courses with whole activities of gamification and discussion rather than listening to talks. Originality/value: For its implications, this study has enriched the literature on gamification implementation for adult learners. Regarding its originality, it has discussed an old issue of Knowles' andragogical principle from the novelty angle of gamification.