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Abstract Engineered living microtissues such as cellular spheroids and organoids have enormous potential for the study and regeneration of tissues and organs. Microtissues are typically engineered via self‐assembly of adherent cells into cellular spheroids, which are characterized by little to no cell–material interactions. Consequently, 3D microtissue models currently lack structural biomechanical and biochemical control over their internal microenvironment resulting in suboptimal functional performance such as limited stem cell differentiation potential. Here, this work report on stimuli‐responsive cell‐adhesive micromaterials (SCMs) that can self‐assemble with cells into 3D living composite microtissues through integrin binding, even under serum‐free conditions. It is demonstrated that SCMs homogeneously distribute within engineered microtissues and act as biomechanically and biochemically tunable designer materials that can alter the composite tissue microenvironment on demand. Specifically, cell behavior is controlled based on the size, stiffness, number ratio, and biofunctionalization of SCMs in a temporal manner via orthogonal secondary crosslinking strategies. Photo‐based mechanical tuning of SCMs reveals early onset stiffness‐controlled lineage commitment of differentiating stem cell spheroids. In contrast to conventional encapsulation of stem cell spheroids within bulk hydrogel, incorporating cell‐sized SCMs within stem cell spheroids uniquely provides biomechanical cues throughout the composite microtissues’ volume, which is demonstrated to be essential for osteogenic differentiation.