Several dietary amino acids can influence a range of functions of the brain. The aromatic amino acids (ArAAs; tryptophan, tyrosine, and phenylalanine) are precursors for the neurotransmitters serotonin, dopamine, and noradrenaline. A single meal, depending on its protein and carbohydrate content, can rapidly influence the uptake of amino acids into the brain and modify directly their conversion to neurotransmitters. Dysfunction in the neurotransmitter synthesis is related to the mood disorders. The most studied amino acid tryptophan works as a precursor for serotonin, which is known to play a key role in mood regulation. Lowered serotonin levels are associated with low mood. Subjects with low mood have a heightened preference to sweet carbohydrates that increases tryptophan and serotonin concentrations in the brain. Catecholamines, dopamine, and noradrenaline are synthesized from the dietary amino acid tyrosine and involved in mood regulation. Tryptophan or tyrosine depletion lowers mood in persons with depression or those having vulnerability to depression. The dietary amino acid glutamate is a precursor for gamma-aminobutyric acid (GABA) and acts as a neurotransmitter itself as well. Glutamate and GABA have a role in mood regulation, but the direct link between the dietary intake and the functions in the brain is still elusive. There are also other amino acids, for example, glysine, serine, lysine, methionine, and histidine that participate in brain functions, but their significance to mood is unclear. The timing and contents of a meal may affect mood and alertness via the dietary amino acids. Usually the daily recommendations for the essential amino acids are exceeded in Western countries. However, individuals with a special diet or chronic disease should take care of the good quality sources of amino acids in their diet. In the future, more studies are needed to elucidate the effects of dietary amino acids on mood.