Mark James Williams, MBE  is a Welsh professional snooker player who is a three-time World Champion, winning in 2000, 2003, and 2018. Often noted for his single-ball long potting ability, Williams has earned the nickname “The Welsh Potting Machine”.

Williams has been ranked the world number one player for a total of three seasons in his career (1999–00, 2000–01, and 2002–03). His most successful season to date was 2002–03, when he won the acclaimed treble of tournaments (known as the Triple Crown): the UK Championship, the Masters, and the World Championship. In doing so, he became only the third player, after Steve Davis and Stephen Hendry, to win all three Triple Crown events in one season. In addition, he is the first player (and to date, the only player) to win all three versions of the professional World Championship – the World Snooker Championship, the Six-red World Championship and the World Seniors Championship.

The first left-handed player to win the World Championship,^[Williams has won 24 ranking tournaments overall, including two UK Championships (1999 and 2002), making him fifth on the all-time list. He has also won the Masters on two occasions (1998 and 2003). His form began to decline after his second World Championship title in 2003; he then dropped out of the top 16 following the 2007–08 season, but he regained his place for 2009–10. Williams has compiled over 500 century breaks during his career, as well as two maximum breaks in competition.