Nivolumab is an immunotherapeutic monoclonal antibody (mAb) that is used for the treatment of several types of cancer. The evidence on its use during lactation is lacking. Here, we report on a 39-year-old woman with metastasized melanoma who was treated with 480 mg nivolumab every four weeks during lactation. Breast milk samples were collected over the course of 34 days, including two cycles of nivolumab. The highest measured concentration of nivolumab during the first cycle was 503 ng/mL at day 13. The cumulative relative infant dose (RID) over the first cycle (28 days) was 9.8 %. The highest overall measured nivolumab concentration was 519 ng/mL at day 33, five days after administration of the second nivolumab cycle. Nivolumab seems to accumulate in breast milk over two consecutive cycles, hence the RIDs of consecutive cycles are expected to be higher. To draw further conclusions regarding safety of breastfeeding during nivolumab therapy, more information about the oral bioavailability of nivolumab in newborns, the nivolumab steady-state concentrations in breast milk and its pharmacodynamic effects are needed.