This study examined a game-based screening tool for measuring early mathematical competencies. Since these competencies have been shown to be predictors of later mathematical achievement, assessment tools are required that identify children who are at risk of experiencing developmental difficulties in this area. Game-based approaches have been effective in nurturing early mathematical competencies, but only few studies have examined their potential in assessment. Tasks that address prognostic, relevant competencies in accordance with two developmental models were embedded in a number board game. A sample of 300 German four-to-six-year-olds played the game. We compared the children’s performance with that of standardized tests. The results confirmed the applicability of the approach and indicated satisfactory internal consistency and content as well as concurrent validity. Moreover, the screening tool identified about 80 % of the children whose performance was clearly below average. The authors discuss the advantages and disadvantages of game-based assessment.