Excessive gestational weight gain (GWG) increases the risk of maternal anaemia during pregnancy, but whether it is associated with offspring anaemia has not been investigated. We aimed to prospectively investigate the association of GWG rate in the second/third trimester with infant Hb concentration and anaemia risk. The present study comprised 13 765 infants born during 2006–2009 to mothers who participated in a trial on prenatal micronutrient supplementation. The GWG was calculated by subtracting the maternal weight at enrolment from that at end-pregnancy. The GWG rate was calculated as dividing the GWG by number of weeks between the two measurements and classified into quintiles within each category of maternal BMI. Infant Hb concentrations were measured at 6 and 12 months of age, and anaemia was defined as an Hb concentration <110 g/l. Of the 13 765 infants, 949 (6·9 %) were anaemic at 6 months and 728 (5·3 %) at 12 months. The GWG rate was inversely and linearly associated with the infant Hb concentrations at both 6 and 12 months (P < 0·001 for linearity). Compared with the middle quintile of GWG rate, the highest quintile was associated with an increased risk of anaemia at 6 months (adjusted OR 1·30, 95 % CI 1·07, 1·59) and 12 months (adjusted OR 1·74, 95 % CI 1·40, 2·17). The associations were consistently mediated by maternal anaemia during pregnancy (P < 0·001). In conclusion, excessive GWG rate appears to be associated with an increased risk of infant anaemia, partly independent of maternal anaemia during pregnancy that mediates the association.