Martyn Elwyn Williams (born 1 September 1975) is a Welsh rugby union footballer and one of the great back row forwards of his generation who curently plays for Cardiff Blues and won 76 caps for Wales as an open-side flanker.

Williams, height 6 ft (1.83 m), weight 15 st 8 lb (99 kg), played club rugby for Pontypridd, with whom he won the 1996-7 Welsh league, then moved to Cardiff in 1999. He captained Cardiff from 2002 until 2005, when Rhys Williams took over the role.

After gaining international caps at every junior level he won his first Wales A cap in 1996 and then made the senior side against the Barbarians the same year. His first appearance in the (then) Five Nations Championship was against England in 1998. He captained Wales for the first time against Scotland at Murrayfield in 2003.

He won his 50th Welsh cap against England in the first match of the 2005 Six Nations Championship and played a prominent part in Wales' Grand Slam that year, notably scoring two tries early in the second half against France in Paris when Wales had appeared to be heading for defeat. He was named RBS Six Nations player of the Championship. Williams announced his retirement from international rugby on 1st October 2007, following Wales' early exit from the 2007 Rugby World Cup.

Williams was selected for the British and Irish Lions tours to Australia in 2001 and to New Zealand in 2005. He holds a brown belt in judo and played centre half for the Welsh YMCA U16 soccer side.

Martyn Williams retired from international rugby after the 2007 Rugby World Cup but made a surprise decision to return to international action when he was recalled by new Wales boss Warren Gatland in January 2008. He intends to play for the Cardiff Blues for another couple of seasons before taking up a career in commercial banking.