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The non-receptor protein tyrosine phosphatase (PTP) SHP2, encoded by PTPN11 , plays an essential role in RAS-mitogen-activated protein kinase (MAPK) signaling during normal development. It has been perplexing as to why both enzymatically activating and inactivating mutations in PTPN11 result in human developmental disorders with overlapping clinical manifestations. Here, we uncover a common liquid-liquid phase separation (LLPS) behavior shared by these disease-associated SHP2 mutants. SHP2 LLPS is mediated by the conserved well-folded PTP domain through multivalent electrostatic interactions and regulated by an intrinsic autoinhibitory mechanism through conformational changes. SHP2 allosteric inhibitors can attenuate LLPS of SHP2 mutants, which boosts SHP2 PTP activity. Moreover, disease-associated SHP2 mutants can recruit and activate wild-type (WT) SHP2 in LLPS to promote MAPK activation. These results not only suggest that LLPS serves as a gain-of-function mechanism involved in the pathogenesis of SHP2-associated human diseases but also provide evidence that PTP may be regulated by LLPS that can be therapeutically targeted. • Disease-associated mutations endow SHP2 liquid-liquid phase separation capability • SHP2 LLPS is driven by electrostatic interactions mediated by PTP domain • SHP2 allosteric inhibitors block SHP2 LLPS by locking SHP2 in closed conformation • Mutant SHP2 can recruit and activate WT SHP2 in LLPS to promote MAPK activation Disease-associated mutants of a critical phosphatase in the RAS-MAPK pathway undergo phase separation through a dominant gain-of-function mechanism, explaining how both enzymatically activating and inactivating mutations dysregulate the pathway and can be therapeutically targeted. [ABSTRACT FROM AUTHOR]