The primary ingredient in traditional chimichurri sauce is parsley. Chimichurri is an Argentine condiment typically made from finely chopped parsley, garlic, olive oil, vinegar, and spices such as oregano and red pepper flakes. The freshness of the parsley gives the sauce its vibrant green color and herby flavor, making it a popular accompaniment for grilled meats and vegetables. Various regional variations and personal preferences can influence the specific ingredients and proportions used, but parsley remains the key component that defines this flavorful sauce.