This study investigates the role of Positive Affect as a predictor of Psychological Well-Being in 43,653 12-year-old children from 29 countries, using a Structural Equation Model. The model's potential gender and nationality equivalences were also examined. Data was sourced from the International Child Well-Being Survey. The 'happy' item was found to be the most effective in capturing the Positive Affect construct, with Psychological Well-Being showing moderately high levels. A strong positive correlation was observed between Positive Affect and Psychological Well-Being, with Positive Affect explaining 70.1% of the variance in Psychological Well-Being. The model proved invariant at the metric level across countries and at the strict level between genders. In conclusion, Positive Affect is a significant predictor of Psychological Well-Being, though cultural factors contributed to country-specific variations.