The Oman international, 33, joined the Royals for their pre-season trip to Thailand and played the first 45 minutes of a 2-2 draw against a Thailand All Stars side.
After impressing, boss Steve Clarke has moved quickly to snap up the former Bolton and Wigan shot-stopper, and he told the club’s official website: “Ali is exactly the type of experienced goalkeeper that we need at this time and I’m delighted that he has chosen to join us.
“Ali has good knowledge of the English game both in the Championship and the Premier League. He is a great shot-stopper and has already impressed everyone with his abilities in the shooting sessions during training.
“Alongside Jonathan Bond and Stuart Moore, Ali brings the goalkeeping department to a very good level and the competition for places should ensure a really high standard.”