The main ingredient used in pesto sauce is basil. This aromatic herb gives pesto its distinctive flavor and fresh aroma. Traditionally, basil is combined with ingredients such as pine nuts, garlic, Parmesan cheese, and olive oil to create a vibrant green sauce. Pesto is often used in Italian cuisine, particularly as a pasta dressing, but it can also enhance various dishes, providing a burst of flavor. Its versatility and rich taste make it a beloved staple in cooking around the world.