There is an emerging consensus that spatial thinking is fundamental to later success in math and science. The goals of this study were to design and evaluate a novel test of three-dimensional (3D) mental rotation for 4- to 8-year-old children (N?=?165) that uses tangible 3D objects. Results revealed that the measure was both valid and reliable and indicated steady growth in 3D mental rotation between the ages of 4 and 8. Performance on the measure was highly related to success on a measure of two-dimensional (2D) mental rotation, even after controlling for executive functioning. Although children as young as 5?years old performed above chance, 3D mental rotation appears to be a difficult skill for most children under the age of 7, as indicated by frequent guessing and difficulty with mirror objects. The test is a useful new tool for studying the development of 3D mental rotation in young children.