Previous studies in both typically and atypically developing children have shown that approximate number system (ANS) abilities predict formal mathematical knowledge later on in life. The current study investigated whether playing specially designed training games that targets the ANS system using nonsymbolic stimuli only would improve preschool children’s ANS abilities. Thirty-eight preschool children were randomly allocated to either the training or control group. For 5 weeks, 20 preschoolers (9 girls) in the training group played daily games for 10 min that included guessing and comparing numerosities, whereas 18 control children (6 girls) were involved in interactive picture book reading sessions. Children’s ANS abilities were assessed using a computerized task before and after the training program. An analysis of covariance with posttraining ANS scores as dependent variable and pretraining scores as a covariate showed that the children in the training group had higher ANS abilities after the training, in contrast to children in the control group (p = .012, ηp2 = .171). This study provides evidence that ANS abilities can be improved in preschool children through a daily training program that targets the ANS specifically. These findings provide support for further training programs for preschool children who show mathematical difficulties early on in life.