We developed and evaluated a self-report instrument for preschoolers assessing self-perceived ability, task difficulty, and motivation. 231 3-to 5-year-old children (M = 3.84 years, SD = .49, 112 girls, 119 boys), participated in the interview, and reported also on their self-concept. Parents and teachers reported on children’s symptoms. In a first step we used a person- and a variable-oriented approach to assess associations between self-perceived ability, estimated task difficulty and motivation. In a second step, we aimed to generate evidence for our construct of self-perceived ability based on relationships between the test scores and other measures that are theoretically assumed to be related to self-perceived ability. Results confirm the theory-based relation between self-perceived ability, task difficulty, and motivation, and indicate construct validity. Self-perceived ability proved to mediate the association between task difficulty and motivation and was associated with children’s task mastery and self-concept. However, children’s self-reports were not associated with adult-reported symptoms. The findings extend the discussion on the validity, reliability, and accuracy of self-reports in preschoolers. Further studies need to address the criterion-related validity of the interpretation of the newly developed instrument.