The Chinese invented ke-tsiap--a concoction of pickled fish and spices (but no tomatoes)--in the 1690s.
By the early 1700s its popularity had spread to Malaysia, where British explorers first encountered it. By 1740 the sauce--renamed ketchup--was an English staple, and it was becoming popular in the American colonies.
Tomato ketchup wasn't invented until the 1790s, when New England colonists first mixed tomatoes into the sauce. It took so long to add tomatoes to the sauce because, for most of the 18th. Century, people had assumed that they were poisonous, as the tomato is a close relative of the toxic belladonna and nightshade plants.
(from http://www.westegg.com/etymology/)
