Twin-bearing ewes may be unable to produce enough colostrum following birth to support the survival of two lambs. This causes the newborn lambs to be at risk of death due to starvation, mismothering, cold exposure and infection. Niacin has previously been demonstrated in cattle to increase immunoglobulin concentrations in colostrum, possibly due to niacin increasing blood flow, allowing the greater accumulation of nutrients and immunoglobulins in colostrum. High producing animals, such as the late gestation twin-bearing ewe, may not be able to produce enough niacin to meet their production requirements. Aims: This experiment evaluated whether supplementation of niacin (vitamin B3) to late gestation twin-bearing ewes would increase colostrum production and composition (concentration of fat, lactose, protein and immunoglobulin G), and postnatal lamb growth. Methods: A total of 24 artificially inseminated Merino ewes were weighed, body condition scored, aged and randomly allocated to a treatment-control treatment or with 5 g/day niacin supplemented for 2 weeks before lambing until 2-8 days post-lambing. Key results: Colostrum production and immunoglobulin G concentration were similar between treatments (P > 0.05). Ewes supplemented with niacin produced significantly higher (P = 0.039) colostral protein at 12.54% compared with the control group ewes at 11.78%. Early lamb growth rates were unaffected (P > 0.05) by treatment. Conclusions: Supplementation of ewes with 5 g/day niacin is not recommended to increase the production of colostrum and immunoglobulin G or to increase lamb growth rates following birth. Implications: Further research into niacin supplementation, supplementation time and supplementation amount for production effects in late gestation ewes is warranted.