Adaptive computer games offer an attractive method for numeracy training in young children. However, the evidence for transfer of learning to standard measures of numerical and arithmetic skills is scarce. We carried out a randomized controlled trial on a sample of preschool children of middle socio-economic status to evaluate the effectiveness of the freeware videogame “The Number Race” (Wilson et al., 2006). Children were randomly assigned to the training group or to the control group performing an alternative computer-based activity matched for duration and setting. The groups were matched for age, gender, and IQ. Training yielded large improvements in mental calculation and spatial mapping of numbers, as well as smaller improvements in the semantic representation of numbers. Our findings complement previous studies that showed beneficial effects for disadvantaged children, thereby suggesting that “The Number Race” is a valuable tool for fostering mathematical learning in the general population of young children.