In February 2009, Williams earned his first call up to the senior Wales squad for the match with Poland in Portugal and was called up a second time the following month for the qualifiers with Finland and Germany. He remained a regular back-up goalkeeper in the following years, including in their successful UEFA Euro 2016 qualifying campaign. He finally made his debut on 13 November 2015 in a friendly against the Netherlands at Cardiff City Stadium, replacing Wayne Hennessey with 16 minutes to play and the score at 2–2, and conceded the winning goal from Arjen Robben. In 2016, he was named in Wales' 23-man squad for UEFA Euro 2016.