What ingredient gives Pad Thai its sour taste?

The ingredient that gives Pad Thai its sour taste is tamarind paste. Tamarind is the fruit of a tropical tree, and its dark, sticky pulp has a pleasantly tart flavor that becomes the dish's characteristic acidic note when diluted into the sauce.

Cooks may use tamarind pulp, tamarind concentrate, or tamarind water rather than a product specifically labeled paste. These forms are culinary equivalents for this question: they all provide tamarind's acidity. The amount varies by recipe, since the fruit's sharpness depends on concentration and sweetness.

Lime wedges are often served alongside Pad Thai and can add fresh acidity at the table, which causes a common mix-up. Lime is a garnish or optional finishing accent in many versions, while tamarind is the traditional souring base in the sauce itself. Vinegar and lemon juice are substitutions, not the canonical ingredient.