The game will be played at the Madejski Stadium on Saturday August 7. Williams joined the Royals on a Bosman free transfer this summer after turning down the offer of a contract to stay at Scunthorpe
Brian McDermott, the Reading manager, was pleased to be at home on the opening day.
"It's nice to be at home for the first game, no matter who it is against. I love playing at home so it's great to be there," he told Reading's official website.
Scunthorpe's first home game will be against Norwich CIty the following Saturday.
The Christmas period could bring two reunions for the club. Boxing Day is a home game against Preston North End, including former Iron favourite Paul Hayes, who also turned down the chance to stay at Glanford Park this summer. United travel to Deepdale on Tuesday October 19.
December 28 brings a trip to Turf Moor to face relegated Burnley, managed by former Iron boss Brian Laws.
A busy festive period will also see an away derby with Doncaster Rovers on New Year's Day and a home game against Watford on January 3.
Once again the Iron will finish the season at Glanford Park. This time Portsmouth will make the long trip from the South Coast on Sunday May 8.
For full list please visit the official Scunthorpe United website.