Background: Consumption of ultra-processed foods [UPFs] may be associated with negative health outcomes. Limited data exist regarding the potential role of UPFs in the occurrence of allergic diseases. The underlying mechanisms underpinning any such associations are also poorly elucidated. Methods: We performed a systematic review and narrative evidence synthesis of the available literature to assess associations between UPF consumption and pediatric allergy outcomes (n = 26 papers), including data on the association seen with the gut microbiome (n = 16 papers) or immune system (n = 3 papers) structure and function following PRISMA guidelines. Results: Dietary exposure to fructose, carbonated soft drinks, and sugar intake was associated with an increased risk of asthma, allergic rhinitis, and food allergies in children. Commercial baby food intake was associated with childhood food allergy. Childhood intake of fructose, fruit juices, sugar-sweetened beverages, high carbohydrate UPFs, monosodium glutamate, UPFs, and advanced glycated end-products (AGEs) was associated with the occurrence of allergic diseases. Exposure to UPFs and common ingredients in UPFs seem to be associated with increased occurrence of allergic diseases such as asthma, wheezing, food allergies, atopic dermatitis, and allergic rhinitis, in many, but not all studies. Conclusion: More preclinical and clinical studies are required to better define the link between UPF consumption and the risk of allergies and asthma. These observational studies ideally require supporting data with clearly defined UPF consumption, validated dietary measures, and mechanistic assessments to definitively link UPFs with the risk of allergies and asthma.