Ethnic discrimination during pregnancy is linked to maternal psychological distress, adverse birth outcomes and increased offspring morbidity and mortality. An evolutionary perspective reframes offspring health issues as a risk to maternal fitness. We argue that kin may be evolutionarily motivated to buffer psychosocial stressors for the mother during pregnancy. Previously, we found that the relationship of a pregnant woman with her own mother (fetus’ maternal grandmother) had a positive association on maternal prenatal psychology, above and beyond her relationship with her fetus’ father. Here, we ask if grandmothers buffer mothers’ prenatal psychological distress from ethnic discrimination. Using self-report data collected from 216 pregnant Latina women living in Southern California, we found discrimination to be significantly, positively associated with depression, anxiety, and stress in linear regression models. Maternal grandmother communication attenuated the association of discrimination and all three psychological distress measures, adjusting for the mother's relationship with the father. Maternal grandmother emotional support similarly significantly moderated the relationship of discrimination with depression and anxiety. We did not observe any significant interactions for paternal grandmother relationships. Geographic proximity was not a significant stress buffer. Results suggest the important role maternal grandmothers play in perinatal mental health, and that these benefits exist uncoupled from geographic proximity.