Glasgow-born Anya, who joined Gianfranco Zola's side from Granada following a loan, has been called up for the first time at international level by Gordon Strachan.
Nottingham Forest striker Jamie Mackie and Leeds United forward Ross McCormack have also been recalled to the squad following Kenny Miller's retirement from international football.
Goalkeeper Allan McGregor is suspended for the first match against Belgium so Strachan has opted to recall Kilmarnock keeper Craig Samson.
"We knew about him at the end of last year and everybody watched him. When any of my friends watched him they passed on information," said the Scotland boss.
"I spoke to him last night and he was delighted.
"He is a late developer. He has been in Spain playing football and I think he was in the Glenn Hoddle foundation out there.
"He is a guy who has seen there is a chance and worked hard."
Anya described his call-up as a privilege and an honour on his Twitter account.
"I'm really happy," he said. "The gaffer (Gianfranco Zola) told me at breakfast one morning that he had been speaking with Gordon Strachan. I was so pleased.
"I'm under no illusion, this call-up would not have been possible without Watford Football Club.
"I'm happy at Watford and hopefully that is evident to see on the pitch. I would like to thank all the staff - coaching, medical and everyone else at the club - for their help."
McCormack scored on his last Scotland appearance, a 3-1 friendly win over Australia a year ago, but illness and injury deprived him of a chance under Strachan.
Strachan said: "He has got pace, technically he is good, he keeps the ball, so they are good credentials for an international player."
"He's had a good start to the season and been big in games. He was unfortunate not to be in the last squad, we felt we would try to keep the squad that was in Croatia together.
"The whole group did so well in the preparation and playing of that game."