Early spatial skills predict the development of later spatial and mathematical skills. Yet, it is unclear how comprehension of the words that capture spatial relations, words like behind and under, might be associated with children's early spatial and mathematics skills. The current study addressed this question by conducting a moderated mediation model to test the potential moderating effects of group factors, such as socioeconomic status (SES) and gender, on the possible mediation of spatial language comprehension on the association between spatial skill and mathematics performance. In total, 192 3-year-olds were tested on a battery of assessments, including a novel Spatial Language Comprehension Task, a test of spatial skills (2- and 3-dimensional trials of the Test of Spatial Assembly [2D and 3D TOSA, respectively]), and a composite of 2 mathematical assessments. The results indicate that this novel Spatial Language Comprehension Task is a reliable measure useful for examining group differences and the early space--math link. Specifically, higher-SES preschoolers and females had higher spatial language comprehension compared with their lower-SES peers and males, respectively. These SES and gender disparities in spatial language comprehension are concerning, given the strong association between spatial language comprehension and mathematics skills. Additionally, spatial language comprehension mediated the association between spatial skill and mathematics performance for females only. Future work should examine the potential causal role that spatial language comprehension may have in concurrent and later spatial and mathematics skills.