Background Uniform international measurement tools for assessing healthy ageing are currently lacking. Objectives The study assessed the novel comprehensive global Ageing Trajectories of Health: Longitudinal Opportunities and Synergies (ATHLOS) Healthy Ageing Scale, using an Item Response Theory approach, for evaluating healthy ageing across populations. Design Pooled analysis of 16 international longitudinal studies. Setting 38 countries in five continents. Subjects International cohort (n=355 314), including 44.4% (n=153 597) males, aged (mean±SD) 61.7±11.5 years old. Methods The ATHLOS Healthy Ageing Scale (including 41 items related to intrinsic capacity and functional ability) was evaluated in a pooled international cohort (n=355 314 from 16 studies) according to gender, country of residence and age group. It was also assessed in a subset of eight cohorts with ≥3 waves of follow-up assessment. The independent samples t-test and Mann-Whitney test were applied for comparing normally and skewed continuous variables between groups, respectively. Results The ATHLOS Scale (range: 12.49-68.84) had a mean (±SD) value of 50.2±10.0, with males and individuals >65 years old exhibiting higher and lower mean scores, respectively. Highest mean scores were detected in Switzerland, Japan and Denmark, while lowest in Ghana, India and Russia. When the ATHLOS Scale was evaluated in a subset of cohorts with ≥3 study waves, mean scores were significantly higher than those of the baseline cohort (mean scores in ≥3 study waves vs baseline: 51.6±9.4 vs 50.2±10.0; p<0.01). Conclusions The ATHLOS Healthy Ageing Scale may be adequately applied for assessing healthy ageing across populations.