Catsup and ketchup are two spellings of the same word for the same sauce. Both are correct, but ketchup is by far the more common spelling today, so use it unless you are naming a product that is sold as catsup.
People often wonder which of the two words to use, what the difference between them is, and why they are spelled differently. Some even say that ketchup tastes different from catsup, or that the two are made from different ingredients. Is that true? No: it is one condiment under two names, and the history of the word shows how that happened.
Where the word comes from
Ketchup is a favorite condiment around the world, but it did not start out as a tomato sauce. The word most likely comes from kê-chiap, the name of a sauce made from pickled fish in the Hokkien language of Fujian, a coastal region of southern China. A related word, kicap, is still the Malay word for soy sauce.
British traders came across the sauce in Southeast Asia in the late 17th century, liked it, and brought the idea home. The word appears in English as catchup in 1690 and as ketchup in 1711, and catsup followed in the 1730s.
Early English ketchups were made from all kinds of ingredients, such as mushrooms, walnuts, oysters, and anchovies. Most people are surprised to learn that tomatoes came in only about a century later, when American cookbooks of the early 1800s printed the first recipes for tomato ketchup. The tomato version soon became very popular in the United States.
How catsup lost to ketchup
For a long time, both spellings were in use, and catsup was common, especially in the United States. Then, in 1876, the H. J. Heinz Company began to sell tomato ketchup, and it was soon sold across the country. By most accounts, Heinz first called its sauce catsup but switched to ketchup in the 1880s, to stand out from its competitors. As Heinz came to lead the market, its spelling spread with it. Today, almost everyone who eats ketchup knows the name Heinz.
So Heinz played a big part in the decline of catsup. You can still use the word, and some brands and some people still do, but most people say and write ketchup instead.
Which spelling should you use?
Both spellings are correct. Catsup is a spelling variant of ketchup, and dictionaries list both, along with the older catchup. In your writing, use ketchup, the spelling most readers expect. Use catsup only when you refer to a product that carries that name on its label.
For more words that people often mix up, see farther vs. further and principle vs. principal.