Tonkatsu sauce is a thick sauce served with tonkatsu, the Japanese pork cutlet. It is a thick (viscosity over 2.0 pascal-second, per JAS standard) Japanese Worcestershire-type sauce. As with most Japanese Worcestershire sauces, it is vegetarian and similar to a brown sauce, with tomatoes, prunes, dates, apples, lemon juice, carrots, onions, and celery among its ingredients.
The first tonkatsu sauce was made in 1948 by Oliver Sauce Co., Ltd. of Hyogo Prefecture. The Bull-Dog brand of tonkatsu sauce, for example, is made from malt vinegar, yeast, and vegetable and fruit purees, pastes, and extracts.  In the United States, Kikkoman brand sells a fruity tonkatsu sauce with applesauce as the main ingredient.