A new approach for imposing long-range, defect-free confinement was used to study the crystallization kinetics of ultrathin polymer layers. Layer multiplying coextrusion was utilized to create films in which hundreds or thousands of continuous microlayers or nanolayers of a crystallizable polymer were separated by layers of a rigid confining polymer. Even in confined layers as thin as a single lamella, isothermal crystallization of poly(ethylene oxide) followed the conventional habit whereby chains folded back and forth in the crystal with a common fold surface free energy and fold length. However, the crystallization rate was substantially suppressed even when the layer thickness was tens of micrometers, a thickness scale where the well-known substrate effect must have been negligible. The crystallization kinetics was modeled by considering the effect of truncation when the growing spherulite encountered the interface. Although crystallization was dramatically retarded as the layers became thinner, the Avrami equation, expressed for heterogeneously nucleated bulk crystallization of polymers, was valid for thin and ultrathin confined films.