Cancer cells use different modes of migration, including integrin-dependent mesenchymal migration of elongated cells along elements of the 3D matrix as opposed to low-adhesion-, contraction-based amoeboid motility of rounded cells. We report that MDA-MB-231 human breast adenocarcinoma cells invade 3D Matrigel with a characteristic rounded morphology and with F-actin and myosin-IIa accumulating at the cell rear in a uropod-like structure. MDA-MB-231 cells display neither lamellipodia nor bleb extensions at the leading edge and do not require Arp2/3 complex activity for 3D invasion in Matrigel. Accumulation of phospho-MLC and blebbing activity were restricted to the uropod as reporters of actomyosin contractility, and velocimetric analysis of fluorescent beads embedded within the 3D matrix showed that pulling forces exerted to the matrix are restricted to the side and rear of cells. Inhibition of actomyosin contractility or β1 integrin function interferes with uropod formation, matrix deformation, and invasion through Matrigel. These findings support a model whereby actomyosin-based uropod contractility generates traction forces on the β1 integrin adhesion system to drive cell propulsion within the 3D matrix, with no contribution of lamellipodia extension or blebbing to movement.