The aim of our study is to describe and understand the experiences of nurses providing emergency care to undocumented migrants who arrive in Spain in small boats. Background: Spain receives thousands of undocumented migrants every year who arrive by sea. Provision of appropriate emergency care to undocumented migrants is a public health problem. Introduction: Nurses, along with other health care providers, such as doctors or cultural mediators, make up the Spanish Red Cross Emergency Response Team. Nurses, in particular, are associated with all phases of emergency care to undocumented migrants who arrive in small boats, offering first aid as well as clinical and humanitarian care. Methods: Qualitative descriptive study. Seventeen nurses from the Spanish Red Cross Emergency Response Team participated in face-to-face interviews. Thematic analysis was used to analyse the qualitative data. Findings: Three main themes emerged: (i) guaranteeing comprehensive emergency care, (ii) the nurse, the key member of the multidisciplinary care team for undocumented migrants and (iii) 'making a difference' , volunteering as a nurse's role. Conclusions: Nurses try to guarantee comprehensive care provision for undocumented migrants, even though they face stigma, ethical concerns or an impossibility to prescribe pharmacological treatments. Personalised care, more time and protocols, better training and the incorporation of debriefing are elements that are required to improve the emergency care given to undocumented migrants. Implications for nursing and health policy: Institutions must develop policies to support provsion of emergency care to undocumented migrants. A public health issue cannot depend upon volunteer healthcare providers. Governments must guarantee funding, training and established care teams. Understanding nurses' experiences could increase awareness of the problem, reduce stigma and improve the comprehensive emergency care provided to undocumented migrants.