Background: To ascertain the construct validity and reliability of a self-administered web-based assessment of intrinsic capacity (IC). The study design was a cross-sectional analysis using data from a prospective cohort. Methods: We included data from 6,434 respondents (mean age 65.33±5.81 years; 52.4% women) of the Singapore Life Panel population study who participated in the online surveys in March 2022 and May 2022. Incremental nested factor structures of IC were modelled with confirmatory factor analysis (CFA) and their goodness-of-fit were assessed mainly with root mean square error of approximation (RMSEA), comparative fit index (CFI), Tucker-Lewis index (TLI), and standardized root mean square residual (SRMR). With the most parsimonious model as our eventual factor structure, we further evaluated IC and its domains with reliability indices. Results: CFA demonstrated construct validity for the second-order factor structure with acceptable overall model fit: χ2(147)=7,696.276, p<0.001; CLI=0.947; TLI=0.938; RMSEA=0.089; SRMR=0.051. Amongst the domains, vitality had highest factor loading (0.889), whereas locomotion and cognition (0.534 and 0.601, respectively) had lowest loadings with the second-order IC factor. All five IC domains and the general IC factor fulfilled reliability thresholds (construct validity [CR] or hierarchical omega ≥0.7; average variance extracted ≥0.5); psychological and locomotion domains have high CR (>0.9), whereas vitality and sensory domains have lower values of CR. Conclusion Our study provides proof-of-concept evidence regarding the construct validity and reliability of a self-administered web-based assessment of IC index that can potentially be scalable in other population settings.