Background: A global campaign to eradicate ‘child marriage’ (<18 years) increasingly targets governments, the private sector and the general public as agents of change. Here, we measure the current state of public knowledge of child marriage, identifying potential misconceptions and inaccurate beliefs. We then consider the implications of our survey for global health initiatives addressing child marriage. Methods: We surveyed USA nationals via the online survey platform MTurk. Participants provided sociodemographic information and answered ten questions about their understanding of child marriage. To identify potential misconceptions and inaccurate beliefs we present descriptive statistics for participant responses paired with correct answers where available. Results: 609 participants provided data for analysis. 59% were women, with a mean age of 37 years, just over half (55%) were university educated, and the majority (79%) were currently employed. Half of those surveyed mistakenly believed that the cut-off for child marriage is younger than the legal threshold of 18 years, and nearly three-quarters incorrectly believed that most child marriages occur at 15 years or below (child marriage primarily takes place in later adolescence). The large majority of participants incorrectly believed that child marriage is illegal throughout the United States (it’s illegal in only two states), and mistakenly believed that child marriage primarily takes place among Muslim-majority regions of the Middle East and North Africa (it is most common in sub Saharan Africa and South Asia). Participants tended to substantially overestimate the prevalence of child marriage in both the USA and abroad. Conclusions: These findings suggest that public understanding of child marriage is not only poor, but also shaped by wider misperceptions of both relatively high and low-income nations. Organizations seeking to empower girls and young women by reducing child marriage need to be cautious of these misunderstandings, and wary of the potential for their own activities to seed misinformation. For example, we suggest the terminology ‘child marriage’, as opposed to ‘teen marriage’ or ‘adolescent marriage’, may contribute to widespread confusion about the typical ages at which child marriages occur.