Jack William Nicklaus (; born January 21, 1940), nicknamed "the Golden Bear", is an American retired professional golfer and golf course designer. He is widely considered to be one of the greatest golfers of all time. He won 117 professional tournaments in his career. Over a quarter-century, he won a record 18 major championships, three more than second-placed Tiger Woods. Nicklaus focused on the major championships—the Masters Tournament, U.S. Open, Open Championship and PGA Championship—and played a selective schedule of regular PGA Tour events. He competed in 164 major tournaments, more than any other player, and finished with 73 PGA Tour victories, third behind Sam Snead (82) and Woods (82).

Nicklaus won the U.S. Amateur in 1959 and 1961 and finished second in the 1960 U.S. Open, two shots behind Arnold Palmer. Nicklaus turned professional at age 21 in 1961. He earned his first professional victory at the 1962 U.S. Open, defeating Palmer by three shots in a next-day 18-hole playoff and launching a rivalry between the golf superstars. In 1966, Nicklaus became the first player to win the Masters Tournament two years running; he also won The Open Championship, becoming at age 26 the youngest player to win all four golf majors. He won another Open Championship in 1970.

Between 1971 and 1980, Nicklaus won nine more major championships, overtook Bobby Jones’ record of 13 majors, and became the first player to complete double and triple career grand slams. He won the 1986 Masters, his 18th and final major championship at age 46, the tournament's oldest winner. Nicklaus joined the Senior PGA Tour (now known as the PGA Tour Champions) when he became eligible in January 1990, and by April 1996 had won 10 tournaments, including eight major championships despite playing a very limited schedule. He continued to play at least some of the four regular Tour majors until 2005 when he made his final appearances at the Masters Tournament and The Open Championship.

Today, Nicklaus heads Nicklaus Design, one of the world's largest golf course design and construction companies. Nicklaus runs an event on the PGA Tour, the Memorial Tournament, named after the annual honoring it bestows to individuals associated with the game of golf. Nicklaus's books vary from instructional to autobiographical, with his Golf My Way considered one of the best instructional golf books of all time; the video of the same name is the best-selling golf instructional to date.