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Pericytes are multipotent mesenchymal precursor cells that demonstrate tissue-specific properties. In this study, by comparing human adipose tissue- and periosteum-derived pericyte microarrays, we identified T cell lymphoma invasion and metastasis 1 (TIAM1) as a key regulator of cell morphology and differentiation decisions. TIAM1 represented a tissue-specific determinant between predispositions for adipocytic versus osteoblastic differentiation in human adipose tissue-derived pericytes. TIAM1 overexpression promoted an adipogenic phenotype, whereas its downregulation amplified osteogenic differentiation. These results were replicated in vivo, in which TIAM1 misexpression altered bone or adipose tissue generation in an intramuscular xenograft animal model. Changes in pericyte differentiation potential induced by TIAM1 misexpression correlated with actin organization and altered cytoskeletal morphology. Small molecule inhibitors of either small GTPase Rac1 or RhoA/ROCK signaling reversed TIAM1-induced morphology and differentiation in pericytes. In summary, our results demonstrate that TIAM1 regulates the cellular morphology and differentiation potential of human pericytes, representing a molecular switch between osteogenic and adipogenic cell fates.