In semi-extensive cattle systems, cows may be exposed to undernutrition, which may reduce the productivity of beef herds. The objective of this study was to evaluate the effects of undernutrition and supplementation with hydroxytyrosol (HT, polyphenol derived from olive leaves) during the last third of gestation on cow productive, metabolic and hormonal profiles, transfer of passive immunity to calves via colostrum, and birth weight and vitality of newborn calves. A total of 109 pregnant beef cows (Parda de Montaña, n = 63; and Pirenaica, n = 46) were assigned into four feeding groups following a 2 x 2 factorial design, according to feeding level (100% vs. 60% of nutritional requirements) and dietary HT supplementation (Control vs. HT; 0 and 180 mg HT/kg total mixed ration) between 28 and 40 week of gestation. At week 28, all groups (100-Control, 100-HT, 60-Control and 60-HT) were balanced in BW, BCS (body condition score), breed and age. Cow BW and BCS were recorded, and plasma concentrations of non-esterified fatty acids (NEFA), cholesterol, insulin-like growth factor 1 (IGF-1), glucose, fructosamine and urea were analysed every three weeks during the last third of gestation. Cow cortisol was measured in late pregnancy and shortly after parturition. Colostrum and plasma from cows and calves were analysed for immunoglobulin (IgM, IgG) concentrations. At birth, calf weight and a vitality test were recorded. Undernutrition in the last third of pregnancy reduced dam BW at parturition and induced fat mobilisation. Hydroxytyrosol supplementation in the 60-HT group led to a more moderate increase in NEFA and cholesterol levels, along with a lesser decline in IGF-1 levels. Hydroxytyrosol also decreased dam plasma urea levels. Calves from HT-supplemented dams were heavier at birth. Undernutrition during the last third of gestation resulted in elevated plasma cortisol levels in newborn calves, which persisted for one month, while dam cortisol levels were not affected. Plasma IgG concentrations were higher in HT-supplemented dams regardless of feeding level, and higher concentrations of IgM and IgG in colostrum were maintained throughout the first 24 hours postpartum on them. Calf vitality was only affected by calving difficulty. Overall, undernutrition during the last third of gestation had negative effects on dam BW, BCS and indicators of metabolic status, partially alleviated by HT supplementation, and increased cortisol levels in newborn. Moreover, HT supplementation increased calf birth weight and helped to maintain colostrum IgM and IgG concentration during the first 24 hours after calving.