This study focuses on three main goals: First, 3‐year‐olds' spatial assembly skills are probed using interlocking block constructions (N = 102). A detailed scoring scheme provides insight into early spatial processing and offers information beyond a basic accuracy score. Second, the relation of spatial assembly to early mathematical skills was evaluated. Spatial skill independently predicted a significant amount of the variability in concurrent mathematical performance. Finally, the relation between spatial assembly skill and socioeconomic status (SES), gender, and parent‐reported spatial language was examined. While children's performance did not differ by gender, lower SES children were already lagging behind higher SES children in block assembly. Furthermore, lower SES parents reported using significantly fewer spatial words with their children.