Background: Despite the importance of lifelong learning as a key to individual and societal prosperity, we know little about adult motivation to engage in learning across the lifespan. Building on educational psychological approaches, this article presents a measure of Motivation-to-Learn using four items from the background questionnaire of the Programme for the International Assessment of Adult Competencies (PIAAC). Methods: We used multiple-group confirmatory factor analyses for ordered categorical data to investigate the scale's dimensionality and measurement invariance across countries. Regression analyses were used to investigate the scale's criterion validity. Results: Results show that the proposed four-item scale fits the data considerably better than the original six-item scale labeled Readiness-to-Learn. Further analyses support the scale's configural, metric (weak) and partial scalar (strong) measurement invariance across 21 countries. As expected, Motivation-to-Learn has significant relations to the working population's engagement in learning in terms of participation in non-formal education over the last 12 months. These relations remain relevant after taking literacy as an indicator of level of education into account. Conclusion: The Motivation-to-Learn scale presented here may be used to indicate adult motivation in cross-country comparisons. The potential of using the scale in future PIAAC analyses and research on adult learning is discussed. [This paper has been presented at the 8th Biennnal SELF Conference, August 20-24, 2015, in Kiel, Germany, and at the 1st International CIDER Conference, January 24-26, 2016, Berlin, Germany.]