A bookmaker, or a bookie, is an organisation or a person that takes bets and may pay winnings depending upon results and, depending on the nature of the bet, the United States, with Singapore and Canada, the only legal bookmaker is state_owned and operated. In Canada, this is part of the lottery program and is known as Sport Select.

Most bookmakers in the USA bet on college and professional sports, though in the UK they offer a wider range of bets, notably on political elections. The probability that it will snow on Christmas day is another common event for betting in the UK.

By adjusting the odds in his favour or by having a point spread, the bookmaker will aim to guarantee a profit by achieving a 'balanced book', either by getting an equal number of bets for each outcome, or (when he is offering odds) by getting the amounts wagered on each outcome to reflect the odds. When a large bet comes in, a bookmaker can also try to lay off the risk by buying bets from other bookmakers. The bookmaker does not generally attempt to make money from the bets themselves, but rather balancing the odds so that the bookmaker makes what is essentially a transaction fee of a few percent.

Bookmaking is commonly found online.

One of the most infamous illegal bookmakers was Robert Angleton of Houston, Texas. Not only was he a bookie, but he also was a police informant about his smaller rivals. When they were shuttered, he took their business. His bookmaking scheme ended with the death of his wife, Doris Angleton.

See gambling, fixed_odds gambling, bet exchange.