The present study compared Japanese and U.S. preschool children's knowledge of geometric shapes. The main goal was to explore if differences in shape-naming conventions in Japanese and English could explain differences in children's understanding of geometric shapes. In ancient Chinese-based languages (e.g., Japanese), all standard 2D shapes (except for the rectangle) take the form of "[the number of angles]-angle-shape" (e.g., "five-angle shape" for pentagon). Not only are the shape labels relatively easy to construct, each label includes a description of a defining property within its name (i.e., the number of angles). In contrast, English shape names are relatively more complex and do not readily suggest defining shape characteristics. A language in which shape names are relatively easy to construct and communicate defining shape properties should boost children's shape knowledge. Participants were 33 Japanese (M[subscript age] = 54.24 months, SD = 7.33 months) and 26 U.S. children (M[subscript age] = 55.58 months, SD = 8.03 months) between 42 and 68 months old. Children were asked to determine the validity of shape stimuli in a Shape Identification Task. They were also given a mental rotation task to control for spatial reasoning. Findings indicated that language-group membership and age, but not spatial reasoning, significantly predicted shape knowledge. As expected, Japanese children outperformed U.S. children in identifying geometric shapes. The findings suggest that labeling and describing geometric shapes using their defining properties could support children's shape learning.