A central aim of Physical Education (PE) is the promotion of basic motor competencies ("Motorische Basiskompetenzen" [MOBAK]). These are the necessary prerequisites for developing a physically active lifestyle. Valid test instruments are needed for the evaluation of the effect of PE. For this purpose, we developed a test instrument for the assessment of basic motor competencies in first graders. We empirically investigated the construct validity of this MOBAK test instrument in a study (N = 317, M = 7.0 years, girls: n = 173, boys: n = 144). The exploratory factor analysis shows a two factorial structure (locomotion, object control), which is confirmed in a confirmatory factor analysis (CFI = 0.96, RMSEA = 0.036, WRMR = 0.65). In subsequent analyses, we were able to demonstrate that the calculation of a factor sum value for each factor is statistically valid, the factorial structure is the same for boys and girls (?[superscript 2] [6] = 6.95, p = 0.33), and no differential item functioning exists. The MOBAK test instrument is sufficient for the test-theoretical requirements and is thus suitable for the evaluation of the effect of PE.