The worry or concern over confirming negative gender group stereotypes, called stereotype threat, is one explanation for women's worldwide underrepresentation in undergraduate science classes and majors. But how does stereotype threat translate into fewer women motivated for science? In this quantitative study with a sample from the US, we use Expectancy Value Theory to examine whether and how stereotype threat concerns might influence women's science identification. To do this, we collected survey data from 388 women enrolled in introductory physics (male-dominated) and biology (female-dominated) undergraduate laboratory classes at three universities. We examined multiple indirect effect paths through which stereotype threat could be associated with science identity and the associated future motivation to engage in scientific research. In addition to replicating established expectancy-value theory motivational findings, results support the novel prediction that one route through which stereotype threat negatively impacts women's science identity is via effects on perceptions about the communal utility value of science. Especially among women in physics who expressed greater stereotype threat concerns than women in biology, science identification was lower to the extent that stereotype threat reduced how useful science was seen for helping other people and society. Implications for ways to create an inclusive learning context that combats stereotype threat concerns and broadens undergraduate women's participation in science are discussed.