Student engagement is widely recognized as a multidimensional construct; however, validated instruments that capture engagement within specific language-task contexts remain limited. Grounded in Egbert et al.’s (2021) five-dimensional model of language task engagement, this study aimed to validate a questionnaire measuring behavioral, cognitive, emotional, social, and agentic engagement in English as a Foreign Language (EFL) reading-task contexts. The 22-item questionnaire was adapted from an established task engagement scale and refined to align with reading-task contexts. Data were collected from 264 undergraduate EFL students at a Vietnamese university. Exploratory factor analysis (EFA) supported a five-factor structure after the removal of one reverse-coded item, with the retained factors accounting for 53.83% of the total variance. Confirmatory factor analysis (CFA) demonstrated good model fit (χ²/df = 1.736, CFI = 0.941, TLI = 0.931, RMSEA = 0.053). All standardized factor loadings were significant, and evidence of composite reliability, convergent validity, and discriminant validity was established. The findings confirm that engagement in reading tasks can be represented as five distinct yet related dimensions. This study contributes a validated, task-specific questionnaire that can support future research on engagement in EFL reading classrooms.