Autism spectrum disorder (ASD) could be actively linked to gastrointestinal disorders, selectivity in the food consumption, lactose intolerance, and the possible sensitivity to the proteins of cow milk, especially casein. These issues have made nutritionally adequate, cow milk substitutes of children in follow up of casein-free dietary interventions. The present review aimed to look at the possible alternatives to cow milk with focus given to nutritional composition, digestibility, allergenicity, environmental sustainability, and practicality to children living with ASD. A narrative review of 39 peer-reviewed studies published between 2003 and 2025 was carried out via Google Scholar, PubMed, Scopus, as well as, Science Direct. Key terms in the search were: casein protein of cow’s milk, plant-based milk alternatives, milk substitutes, and specific alternatives like soy, almond, oat, rice, pea, and coconut milk. The results suggest that plant-based and select animal-derived milk alternatives can be used to support casein-free dietary management; though, the nutritional value of these milk alternatives differ significantly. Most soy and pea-based milks tend to have higher protein content, whereas most nut, cereal, seed, and pseudo-cereal based milks may require dietary fortification or complementary foods to support the intake needs of calcium, vitamin D, vitamin B12, and essential amino acids. Proper derived processing techniques such as fermentation process, sprouting, cooking, enzyme hydrolysis may enhance digestibility and nutrient bioactivity. Careful dietary planning and professional supervision are recommended.