The Utrecht Work Engagement Scale (UWES) is a self-report instrument widely used, both in the original and its abbreviated version of nine items, to assess the work -UWES, UWES–9– and academic engagement -UWES-S, UWES–S–9–. The present study examines factor structure of the UWES–S–9 using confirmatory factor analysis (CFA), its convergent validity and invariance across sex and age groups in a sample of 626 Spanish high school students. The results support an unidimensional conceptualization of engagement (S-Bχ2/df = 5.29; CFI = .96; NNFI = .94; RMSEA = .083; IFI = .96; AIC = 82.21; BIC = 267.38), revealed an essentially invariant structure of the UWES–S–9 across the sex, ΔS-Bχ2(Δ6) = 10.67; p ≤ .05, and age, ΔS-Bχ2(Δ7) = 9.67; p ≤ .05, and confirmed the positive association between academic engagement and achievement (r = .30; p ≤ .001), dispositional optimism (r = .21; p ≤ .001), and subjective well-being (r = .16; p ≤ .001), and the negative association with perceived stress (r =. –13; p ≤ .001). In short, the Spanish version of UWES–S–9 is presented as a brief, reliable and valid tool to measure academic engagement in high school students (α = .91, AVE = .52, Ω = .911).