Neutrophil extracellular traps (NETs), made of processed chromatin bound to cytotoxic enzymes, are released by neutrophils into the extracellular space to control microbial infections. However, NETs have also been implicated in cancer metastasis. Here, Park et al. observe NET formation in a mouse model of metastatic breast cancer and in clinical samples of primary tumour and metastatic lung lesions from individuals with breast cancer. In in vitro models, NET formation promoted cancer cell migration and invasion; in a mouse breast cancer model, daily intraperitoneal treatment with NET-digesting DNase I-coated nanoparticles inhibited lung metastasis.