Thomas Crapper (baptized 28 September 1836; died 27 January 1910) was a plumber who founded Thomas Crapper & Co in London. Contrary to widespread misconceptions, Crapper did not invent the flush toilet. He did, however, do much to increase the popularity of the toilet, and developed some important related inventions, such as the ballcock. He was noted for the quality of his products and received several royal warrants.

Manhole covers with Crapper's company's name on them in Westminster Abbey are now one of London's minor tourist attractions. Thomas Crapper & Co owned the world's first bath, toilet and sink showroom, in King's Road until 1966. The firm's lavatorial equipment was manufactured at premises in nearby Marlborough Road (now Draycott Avenue).

Crapper held nine patents, three of them for water closet improvements such as the floating ballcock, but none was for the flush toilet itself. Thomas Crapper's advertisements implied the siphonic flush was his invention; one having the text "Crapper's Valveless Water Waste Preventer (Patent #4,990) One movable part only", but patent 4990 (for a minor improvement to the water waste preventer) was not his, but that of Albert Giblin in 1898. Crapper's nephew, George, did improve the siphon mechanism by which the water flow is started. A patent for this development was awarded in 1898.

It has often been claimed in popular culture that the slang term for human bodily waste, "crap", originated with Thomas Crapper because of his association with lavatories. The most common version of this story is that American servicemen stationed in England during World War I saw his name on cisterns and used it as army slang, i.e. "I'm going to the crapper."

The word crap is actually of Middle English origin; and predates its application to bodily waste. Its most likely etymological origin is a combination of two older words, the Dutch krappen: to pluck off, cut off, or separate; and the Old French crappe: siftings, waste or rejected matter (from the medieval Latin crappa, chaff). In English, it was used to refer to chaff, and also to weeds, or other rubbish. Its first application to bodily waste, according to the Oxford English Dictionary, appeared in 1846 under a reference to a crapping ken, or a privy, where ken means a house.