William Todd Hamilton (born October 18, 1965) is an American professional golfer.

Hamilton was born in the small west-central Illinois city of Galesburg. He grew up in an even smaller town, Oquawka, in Henderson County on the Mississippi River. He attended Union High School in Biggsville, Illinois. He attended the University of Oklahoma, where he played collegiately.

Hamilton turned professional in 1987 but was unable to gain entrance to the PGA Tour. Instead he played internationally for many years, primarily on the Japan Golf Tour. When he left the Japan Golf Tour, he was the tour's all-time leading non-Japanese money winner, with earnings of over 630 million yen (about $6.18 million in 2014 US dollars) with 11 tour wins.

After eight tries, at the age of 38, Hamilton went back to Qualifying School in 2003, where he finally earned his first PGA Tour card.

Hamilton won his first PGA Tour event at the 2004 Honda Classic. He birdied the final two holes to beat Davis Love III by one stroke at 12 under par. Later that year, Hamilton won a major championship in one of golf's all-time upsets when he defeated Ernie Els in a four-hole playoff to win The Open Championship at Royal Troon Golf Club. After shooting an opening round 71, Hamilton fired a second round 67 to move to -4 and a fifth-place tie with future World Golf Hall of Famers Els, Vijay Singh and Colin Montgomerie as well as Michael Campbell. Hamilton again shot a 67 in the third round to take a one-shot lead over Els. Entering the tournament's 72nd hole, Hamilton held a one-shot lead over Els, but Hamilton bogeyed the 18th hole, leaving Els with a 12-foot birdie putt for the win, which he missed. Els and Hamilton headed for the four-hole aggregate playoff, in which Hamilton carded four pars while Els managed three pars and a bogey, and Hamilton took the win.

These two victories in his first season on the PGA Tour led to Hamilton being named the 2004 PGA Tour Rookie of the Year. He continued to play well in the wake of the Open Championship win, making the cut in the five remaining tour events he played. However, in his 187 subsequent tour starts, he missed the cut 111 times and had just three top-10 finishes. In 2006, Hamilton captained the American team in ITV's celebrity golf tournament, the All*Star Cup.

He lost his full exempt status on the PGA Tour in 2010. Hamilton played on the Web.com Tour in 2014 and 2015. Hamilton became eligible to play on the Champions Tour after turning fifty years of age in October 2015.

Hamilton lives in Westlake, Texas.