Thousand Island dressing is a variety of salad dressing most commonly made of, primarily, mayonnaise, ketchup, and a mixture of finely chopped vegetables, most often pickles, onions, peppers, and/or green olives; chopped hard-boiled eggs are also common. Thousand Island was invented as a variant upon Russian dressing in the first decades of the 20th century by Sophia LaLonde. The dressing was popularized by one of her dinner guests, actress May Irwin, who also gave the condiment its name, after LaLonde's home, the Thousand Islands region of upstate New York.

George Boldt, of Waldorf=Astoria Hotel fame, has also been widely credited with popularizing this condiment.