This study utilised scale development analysis to evaluate the effectiveness of gamification based on Knowles' andragogical principle in facilitating online learning for adult students. An exploratory sequential mixed-method research design was employed, incorporating interviews and field notes to gather qualitative data for scale construction. The "Gamification for Adult Questionnaires" (GAQ) scale was developed via questionnaires and analysed quantitatively. The thematic analysis of qualitative interviews revealed 23 sub-themes based on the five main andragogical principles used as a foundation for the quantitative analysis scales. Initially, 47 items were constructed, but five items with low communalities were excluded and an EFA with varimax rotation was performed on the remaining 42 items. The KMO and Bartlett test yielded a satisfactory value of 0.90 Williams et al. (2010). The Chi-square test (Bartlett's method) resulted in a significant sphericity level of p < 0.01. The EFA results enabled the categorisation of the 42 items into four factors: independent learning (9 items), learning engagement (15 items), knowledge-experience sharing (7 items), and application to real-life situations (11 items). The high alpha coefficient of 0.97 suggests the scale is reliable, and the 42 items account for 81% of the variance. The results indicate that the GAQ scale is a valid and reliable tool for evaluating gamification based on andragogical principles in adult learning settings. This study provides a foundation for future researchers interested in exploring gamification with contemporary teaching methods and technology for adult learners using Knowles' andragogical principles.