Children's representations of geometric categories like triangles are often centered on a prototypical exemplar (e.g., an equilateral triangle). New cases are judged based on perceptual similarity to the prototype; such a strategy leads to systematic errors in categorization. Creating correct geometric categories requires children to move beyond a reliance on perceptual similarity and learn category-defining rules (e.g., a triangle is an enclosed, three-sided shape). In this research, we test whether a brief training experience using comparison could help three- and four-year-old children learn the category of triangle. Further, we ask whether different types of comparisons (within-category or between-category) support learning in distinct ways. The data indicate that both types of comparison fostered category learning, but that within-category comparisons promoted generalization to new exemplars whereas between-category comparisons reduced overgeneralization to non-exemplars. Furthermore, these effects were moderated by the perceptual similarity of the compared pairs. The results indicate that comparison can foster spatial category learning.