The authors investigate the role of intragroup functional diversity in influencing intergroup relations, defined as intergroup cooperation and group-level citizenship behavior toward other workgroups. Based on predominantly Western literature, the authors hypothesize that groups working in an American culture will show an inverted U-shaped relationship and that groups operating in a Chinese culture will show a J-shaped relationship. The two hypotheses are tested using 67 American workgroups in eight companies and 149 Chinese workgroups in 12 companies. The hypotheses are largely supported with some nuanced departure from the predicted curves. In particular, except for one negative linear relationship, American workgroups and workgroups with highly individualistic values display an inverted J-shape, with both homogenous and moderately diverse groups showing more positive intergroup behaviors than extremely diverse groups. Chinese workgroups exhibit a U-shaped pattern, with both homogenous and highly diverse groups showing the most positive intergroup cooperation. All Chinese groups show a high level of group citizenship behavior regardless of diversity. Collectivistic groups show the J-shape regarding both outcomes, as hypothesized. The article concludes with implications of findings for future research on workgroup diversity and for practices in cross-cultural management.