Mayonnaise is the classic condiment traditionally served with Belgian fries.
Belgian fries, or frites, are commonly sold from frietkoten and other specialized stands. They are typically fried twice, a method intended to produce a crisp exterior and a soft interior. Mayonnaise is the traditional accompaniment, although many vendors offer a long list of additional sauces.
The fries are often served in a paper cone or carton, with the sauce placed on top or in a separate portion. Popular alternatives can include ketchup, samurai sauce, tartar sauce, Andalouse sauce, and many local combinations. Those options are widespread, but mayonnaise remains the standard traditional answer.
Belgium and France both claim important parts of the history of fried potatoes, and the origin story is disputed. That historical debate does not affect the condiment convention: Belgian street fries are especially identified with mayonnaise rather than ketchup.