Evidence continues to accumulate on the detrimental effects of ultra-processed foods (UPFs) on health, environmental sustainability and cultural and economic outcomes, yet UPF sales are rising globally(1). While the UPF concept has gained salience in recent years, emerging evidence shows that consumers may struggle to differentiate between various food processing levels(2). This study aimed to determine how frequently Reddit forum users were able to accurately categorise foods according to their level of processing. A secondary aim was to identify food groups that were commonly miscategorised and ingredients that contributed to confusion or miscategorisation. Data were collected from the subreddit “r/ultraprocessedfood” in January 2025. With >34,000 subscribers, this subreddit is an online community dedicated to discussion of UPFs. To be eligible, threads had to be posted on the “Is this UPF” section of the subreddit and contain a food that could be categorised using Nova. A total of 366 threads were screened, with 288 eligible for inclusion. Accuracy was defined as the correct identification of UPF markers(3). Through content analysis, we assessed the frequency of accurate UPF identification overall and by food category, and the markers most commonly causing confusion. Markers were assessed individually (e.g., lecithin, rapeseed oil) and grouped by Codex additive functional class (e.g., emulsifier) or ingredient type (e.g., vegetable oil). Overall, 176 (61%) products were UPF. The majority of the initial post in the thread asked whether the product was UPF (n=273, 95%). The food categories of products most frequently queried were cereal based products and dishes (n=99, 34%), milk products and dishes (n=24, 8%) and savoury sauces and condiments (n=20, 7%). Overall, the majority of replies accurately categorised foods as either UPF or non-UPF (81%). Categorisation accuracy was high for some food group categories including non-alcoholic beverages (100%), special dietary foods (96%) and milk products and dishes (89%). In contrast, accuracy was lower for seed and nut products and dishes (55%) and savoury sauces and condiments (59%). A misunderstanding about the markers of UPFs was evident in 133 replies, while disagreement about these markers occurred in 57 replies. Markers that caused the most confusion were rapeseed oil, citric acid, seed oils and preservatives. At the group level, confusion regarding markers categorised as vegetable oils (20%), food extracts (11%), preservatives (7%) and acidity regulators (6%) were most common. Overall UPF identification accuracy was high, indicating that among r/ultraprocessedfood members, how to identify UPFs was widely understood. However, these individuals are highly engaged in the topic and likely to possess greater UPF knowledge compared to the general public. The markers that caused most confusion are an important area for future public health messaging to focus on to enhance the public’s ability to accurately identify foods as UPF.