Depression and anxiety are common conditions that greatly hinder daily functioning and aggravate chronic diseases. Recent studies suggest that dietary changes can enhance mood and quality of life without relying on pharmacological therapies. Research indicates that individuals who consume whole foods experience fewer depressive symptoms, and vegetarians tend to have better moods compared to omnivores. Depression and anxiety are associated with brain inflammation and neurotransmitter imbalances. Plant-based diets, which are high in antioxidants and phytochemicals, can help heal brain damage, reduce inflammation, and restore neurotransmitter balance. For example, quercetin from plants acts as a monoamine oxidase (MAO) inhibitor, boosting levels of serotonin (5-HT), dopamine (DA), and norepinephrine (INN). Conversely, arachidonic acid (AA, C20:4n, ω-6), found in animal products, can cause prolonged inflammatory responses linked to depression and anxiety. While increasing ɷ-3 intake is often suggested for mood improvement, evidence points to greater benefits from plant-based diets. Studies have shown that vegetarians experience notable mood improvements compared to those who eat fish or meat. Plant-based diets include foods such as walnuts, flaxseeds, chia seeds, and leafy greens, which provide alpha-linolenic acid (ALA), which the body converts into ɷ-3s, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Additionally, plant-based diets enhance tryptophan (Trp) utilization, which is crucial for 5-HT production. Therefore, a systematic review was conducted according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, searching PubMed and ScienceDirect using several keywords such as “plant-based diets” for studies published from 2017 to 2024. In conclusion, given the increasing prevalence and healthcare costs of these mental health issues, this research aims to examine the impact of dietary patterns, particularly plant-based diets, on mental health, and their potential to reduce symptoms of depression and anxiety through dietary modifications.