The object of this research was to investigate young children's mathematical spatial reasoning in a Montessori classroom. Spatial reasoning is an important part of children's mathematical learning and development; however, opportunities for rich spatial reasoning are not readily available in the classroom. Rather, there is a focus on numeracy at the expense of geometry where activities for spatial development are usually found. Montessori designed a sensory curriculum around children's development, yet spatial reasoning in a Montessori classroom has not been fully investigated. This was a qualitative study using some tools of ethnography. The theoretical framework was Radford's sensuous cognition (2013, 2014) which allowed for an understanding of human development as cultural with the body essential to that development. The data, captured by video, were the children's semiotic traces (Bartolini Bussi and Baccaglini-Frank (2015, p. 393) which are the visible productions of the children's spatial reasoning such as their movements, text, drawings, and speech. The analysis found that the children had ample opportunities for engaging in challenging mathematical problems which required their spatial reasoning. These engaging activities resulted in the children using a wide range of spatial skills as they reasoned mathematically. The children's movement, the main semiotic trace generated by the children, was crucial to their spatial reasoning. This investigation concluded the pedagogical practices created a rich and dynamic environment for the children's spatial development. Practices included the use of well-designed mathematical manipulatives, engagement in the manner of guided play, co-operative learning with peers of mixed ages, extensive time for activities, and assessment based on observations of individual children.