Background: Biological tissues normally possess varying rigidities, changes in which may reflect transformation from normal to pathological state. Cancer cells within the tumor are influenced by the mechanical conditions of their microenvironment, which can drive cell fate. Methods: We present a new on-array methodology to mimic and control desired surrounding rigidity in vitro for three-dimensional (3D) breast cancer object formation and growth. 3D objects were generated from single cells within a hydrogel array, cultured under various mechanical conditions and measured at single-object resolution. Results: Alterations in development of 3D breast cancer microtissue under various rigidity conditions in vitro are revealed. Object growth rate, morphology and vital features are associated with the extent of environmental rigidity, the point in time at which embedding was performed and the initial number of seeded cells. Under stiffness that resembles tumor tissue, higher growth rate of breast cancer microtissue and specific “preinvasive” phenotype are evident. Hallmarks of this phenotype include loose morphology with unclear edges and massive peripheral cell spread, dispersed intrinsic structure and reduced expression of epithelial surface markers. Conclusions: Physical changes in cell environment without parallel changes in biochemical conditions affect growth and development of 3D cancer microtissue grown from individual cells in vitro. A stiffer environment supports 3D microtissue growth and its morphologic and functional diversity in comparison to regular low rigidity conditions.