The interconnected crises of climate change, biodiversity loss, antimicrobial resistance, zoonotic diseases emergence, animal welfare concerns, and the rising burden of non-communicable diseases require food-system responses that extend beyond individual nutrition. This narrative review synthesizes evidence on plant-based and plant-forward dietary transitions within the One Health framework, with emphasis on their implications for human, animal, and environmental health. The reviewed evidence indicates that reducing reliance on animal-derived foods, particularly those from intensive livestock systems, can lower zoonotic spillover risk, reduce antimicrobial use in animal agriculture, and decrease exposure to food-system drivers of antimicrobial resistance. Well-planned plant-based diets may also support cardiometabolic health and reduce risk factors associated with major non-communicable diseases, including cardiovascular disease, type 2 diabetes, obesity, and some cancers. From an environmental perspective, plant-based dietary shifts can reduce greenhouse gas emissions mainly by decreasing demand for ruminant livestock, lowering methane emissions from enteric fermentation, reducing manure-related nitrous oxide emissions, decreasing feed-crop production, and limiting land-use change associated with grazing and animal feed cultivation. They can also reduce pressure on forests, freshwater resources, nutrient pollution, and biodiversity, although these benefits depend on which plant foods replace animal products, how they are produced, their degree of processing, and whether sourcing and land-use policies prevent environmental burden shifting. The review also highlights important challenges, including cultural attachment to meat-based diets, affordability and access, agricultural capacity, nutritional planning, and policy structures that continue to favor animal production. To translate these findings into real-world benefits, plant-forward diets should be implemented through gradual, culturally acceptable dietary changes supported by nutrition education, updated dietary guidelines, public procurement, incentives for legumes and other sustainable plant-based foods, responsible sourcing, and the integration of food policy into climate, biodiversity, antimicrobial resistance, and public health strategies. In this form, plant-based dietary transitions can function as a practical One Health intervention that simultaneously supports human health, animal welfare, and planetary resilience.