Mastery motivation is an important developmental construct that has implications for development across the lifespan. Research to date has focused predominantly on infants and children, with the Dimensions of Mastery Questionnaire (DMQ) being the most widely used measure of mastery motivation. This article reports on the development and initial validation of an adult measure: the Dimensions of Adult Mastery Motivation Questionnaire (DAMMQ). Six hundred and twenty-eight adults (68% female) aged from 18 to 90 years completed the questionnaire. Factor analysis produced 24 items that represented five factors: task persistence, preference for challenge, task-related pleasure, task absorption, and self-efficacy. The DAMMQ was found to have good internal consistency, test-retest reliability and concurrent validity. Within group differences for age, gender and education are reported. The development of the DAMMQ paves the way for future research about mastery motivation in adult populations.