Wales are facing Azerbaijan on Saturday in their Euro 2004 qualifying campaign. The Welsh sit top of their group following a win in Finland and an incredible 2-1 defeat of Italy.
Williams had been tipped by many for a international recall in recent months. He already has twelve caps to his name.
The 31-year-old centre-back left his home at 6am this morning before flying out with the squad.
Boss Alan Pardew was quick to praise his defensive mainstay who has helped the Royals to keep the best defensive record in the Division this season.
Pardew told readingfc.co.uk: "Adie Williams has been called up quite rightly on the back of his perfromances for us this year, and I like to think the team has contributed to his success.
"I'd like to wish Adie, and the Welsh side as well, the best of luck in their journey to the European Championships."
Meanwhile, no date has yet been set for Reading's postponed clash with Sheffield United. However, the Blades have confirmed that all tickets for the original match will be valid for the resceduled fixture.