Williams made his international debut on 5 February 2000 against France in the opening round of the 2000 Six Nations Championship at Millennium Stadium in Cardiff. His first try for Wales came in the next round of the Six Nations against Italy, also at Millennium Stadium. Williams went on to score tries in the 2003, 2007 and 2011 editions of the World Cup. His final try for Wales came literally at the end of his international career, as he scored on the final play of his last Test on 3 December 2011 against Australia at Millennium Stadium. This try was his 28th at Millennium Stadium, surpassing Rory Underwood of England, with 27 at Twickenham, for the most international tries scored by a player from one of the ten "Tier 1" nations at a single ground. Among players from all nations, Williams is level with Ohata, who scored 28 tries at Chichibunomiya Rugby Stadium in Tokyo.