Introduction: Inappropriate fluid therapy may induce or worsen existing hyponatraemia with potentially life-threatening consequences. Nurses have an important role in assisting physicians in IV fluid prescribing. However, research is lacking in Denmark about nurses’ knowledge pertaining to IV fluid therapy and hyponatraemia. Methods: An explorative cross-sectional survey was performed among Danish emergency department nurses in Spring 2019. Knowledge about IV fluid therapy was assessed for three common clinical scenarios, and multiple-choice questions were used to measure knowledge about hyponatraemia. Results: 112 nurses responded to all scenario questions corresponding to 6.2% (112/1815) of the total population of nurses working at emergency departments in Denmark. In two of the three scenarios, a minority of nurses (8–10%) inappropriately selected hypotonic fluids. Nearly one third (31%) selected a hypotonic fluid for a patient with meningitis, which is against guideline recommendations. The study revealed limited knowledge about severe symptoms of hyponatraemia, patients at high risk, and hyperglycaemia-induced hyponatraemia. Conclusion: In accordance with guideline recommendation, the majority of nurses did not select hypotonic fluids in three clinical scenarios commonly encountered in the emergency department. However, when setting up an educational program, further awareness is needed regarding symptoms of hyponatraemia, high-risk patients, and hyperglycaemia-induced hyponatraemia.