Human milk is considered the optimal source of nutrition for infants. Maternal diet is associated with the composition of human milk. The Mediterranean diet (MedDiet) has been studied in pregnancy and during lactation, and it has been associated with changes in milk composition, yet there is a lack of research on MedDiet during lactation and infant outcomes. Mother-infant dyads ( = 167) from ABC Baby, a prospective observational study, were included in this analysis. Maternal diet was obtained using an adapted version of the National Cancer Institute Diet History Questionnaire II, at 2 weeks or 2 months postpartum. Maternal MedDiet score was calculated using servings of vegetables, fruits, whole grains, nuts and seeds, legumes, fish, monounsaturated-to-saturated fatty acid ratio, red and processed meats, and added sugar. Infants' length, weight, and flank skinfold thickness were measured at 6 months. Using World Health Organization standards, weight-for-age (WAZ), length-for-age (LAZ), and weight-for-length (WLZ) Z-scores were calculated. Multiple linear regression models were adjusted for potential confounders. Higher maternal MedDiet score and intake of fruit and fish were associated with lower flank skinfold thickness ( = -0.33, -0.52, and -1.26, respectively). Intake of nuts and seeds was associated with higher WLZ ( = 0.29). Intake of red and processed meats was associated with lower WAZ ( = -0.18) and LAZ ( = -0.18). Energy-adjusted added sugar intake was associated with lower WLZ ( = -0.02). The maternal MedDiet score was associated with lower skinfold thickness, while its components were associated with differences in anthropometric Z-scores. Further research on the maternal MedDiet and corresponding human milk composition is needed to explore this relationship.