Research on scientific reasoning plays an essential role at the intersection of cognitive development and science education. As only few paper-pencil-tests exist to assess scientific reasoning abilities of elementary school children, the goal of the current study was the development of a new reliable and valid instrument. We focused on the assessment of the understanding of the scientific inquiry cycle (SIC) which is a core element of scientific reasoning. As confirmed by IRT modeling, the 15 developed items formed a reliable scale (n = 878). Regression analyses revealed that cognitive abilities (fluid intelligence, reading skills) as well as epistemic beliefs predicted children's SIC performances. The instrument can contribute to the measurement of educational progress in the area of science learning.