Background: Resident-to-resident aggression in nursing homes is a public health problem of growing concern, impacting the safety, health and well-being of all residents involved. Despite this, little research has been conducted on its occurrence particularly in large-scale national studies. The aim of this study was to explore the extent and nature of resident-to-resident aggression in Norwegian nursing homes, as reported by nursing staff. Methods: We conducted a cross-sectional exploratory study, where nursing staff in 100 randomly selected Norwegian nursing homes completed a pen and paper survey measuring how often they had observed incidents of resident-to-resident aggression during the past year. These rates were separated according to nursing home size, location and units of workplace. Results: Of the 3693 nursing staff who participated (response rate 60.1%), 88.8% had observed one or more incidents of resident-to-resident aggression during the past year, with acts of verbal and physical aggression being the most commonly reported. Nursing staff working in dementia special care units, larger nursing homes and nursing homes located in suburban/urban municipalities, reported more incidents of resident-to-resident aggression than staff in short-term and long-term units, small institutions, and nursing homes located in rural municipalities. Conclusions: This is the first national study of resident-to-resident aggression in Norwegian nursing homes and is one of the largest surveys worldwide exploring the extent and nature of resident-to-resident aggression in long-term care settings. Overall, we found a high occurrence of all types of aggression, suggesting a need for strategies to improve residents' safety and quality of life in nursing homes.