Background and aims: Gambling-related harm to concerned significant others (CSOs) is an important public health issue since it reduces CSOs' health and wellbeing in numerous life domains. This study aimed to 1) estimate the first national prevalence of CSOs harmed by gambling in Australia; 2) identify the characteristics of CSOs most at risk of harm from another person's gambling; 3) compare the types and number of harms experienced by CSOs based on their relationship to the person who gambles; and 4) compare the number of harms experienced by CSOs by self-identified gender. Methods: Based on a national CATI survey weighted to population norms, 11,560 respondents reported whether they had been personally and negatively affected by another person's gambling in the past 12 months; and if so, answered detailed questions about the harms experienced from the person's gambling who had harmed them the most. Results: Past-year prevalence of gambling-related harm to adult Australian CSOs was (6.0%; 95% CI 5.6%-6.5%). CSOs most commonly reported emotional harms, followed by relationship, financial, health and vocational harms, respectively. Former partners reported the most harm, followed by current partners, other family members and non-family members, respectively. Female CSOs were more likely to report more harm and being harmed by a partner or other family member, and male CSOs from a non-family member. Discussion and conclusions: The findings provide new insights into the wider societal burden of gambling and inform measures aimed at reducing harm to CSOs from gambling and supporting them to seek help.