What does it mean to be (seen as) human? Ten studies explore this age-old question and show that gender is a critical feature of perceiving humanness, being more central to conceptions of humanness than other social categories (race, age, sexual orientation, religion, disability). Our first six studies induce humanization (i.e. anthropomorphism) and measure social-category ascription. Across different manipulations (e.g. having participants recall experiences, observe moving shapes, imagine nonhuman entities as people, and create a human form), we find that gender is the most strongly ascribed social category and the one that uniquely predicts humanization. To provide further evidence that gender is central to conceptions of personhood, and to examine the consequences of withholding it, we then demonstrate that removing gender from virtual humans (Study 5), human groups (Study 6), alien species (Study 7), and individuals (Study 8) leads them to be seen as less human. The diminished humanness ascribed to nongendered and genderless targets is due, at least in part, to the lack of a gender schema to guide facile and efficient sense-making. The relative difficulty perceivers had in making sense of nongendered targets predicted diminished humanness ratings. Finally, we demonstrate downstream consequences of stripping a target of gender: Perceivers consider them less relatable and more socially distant (Study 8). These results have theoretical implications for research on gender, (de)humanization, anthropomorphism, and social cognition, more broadly.