Background: Ultra-processed foods (UPFs) represent a substantial part of modern diets, with a growing prevalence in food environments worldwide. Their unfavourable nutritional composition and adverse health effects present growing public health concerns. Methods: This study examines the prevalence of UPFs in the Slovenian food supply, their nutritional quality and the use of different food symbols and labelling schemes on food packaging. A cross-sectional analysis was conducted using the representative Slovenian branded foods database. A total of 23,173 prepacked foods and beverages were categorised into levels of processing according to the NOVA classification system. The nutritional composition of UPFs was compared to less processed products within 16 narrow subcategories. Additionally, the prevalence in the use of front-of-package nutrition labelling (FOPNL) and subjectively nutrition-related elements (SNREs) (such as EU Organic, Vegan labels etc.) were assessed across different food categories and processing levels. Results: Results show that UPFs represent 54.5% of the available products in the Slovenian food supply, with the highest prevalence in Confectionery (93%), Bread and bakery products (83%), Meat, meat products and alternatives (77%) and Convenience foods (74%). Comparison of nutritional composition indicated that UPFs had significantly poorer nutritional composition compared to less processed counterparts, including higher levels of sugar, salt and saturated fats, and a lower protein content. Breakfast cereals, Snack foods, Meat alternatives and Pre-prepared salads and sandwiches showed the most significant differences between UPFs and less processed counterparts. Analysis of the prevalence of symbols and labelling schemes revealed that 33.8% of products carried at least one FOPNL (15.0%) or SNRE (19.1%), with SNREs being more prevalent on less processed products and FOPNL predominantly used on UPFs (p < 0.05). The most prevalent SNRE was the EU Organic logo (12.7%), followed by the Vegan (4.7%) and Non-GMO (3.1%) logos, whereas the most frequent FOPNL was Reference Intakes (RI), presenting only energy value RI-Energy (12.5%), followed by nutrient-specific RI (1.6%), while other FOPNL were scarce and limited to certain categories. An additional comparison of visual presentation highlighted the potentially selective use of voluntary FOPNL to improve product framing. This raises concerns about their role in guiding consumer choices versus serving as marketing tools, especially when it comes to UPFs. Conclusions: Our findings highlight the need for monitoring UPFs in the food supply together with harmonised, mandatory labelling regulations to ensure transparency and empower consumers to make healthier choices.