The 26-year-old Glasgow Warriors lock has been called up after Fraser McKenzie injured his arm in Amlin Challenge Cup action for Sale Sharks against Brive on Thursday night.
McKenzie will be seen by a specialist surgeon but has been ruled out of action in the immediate future.
Ryder told the Warriors website: "I'm delighted and proud to have received the call-up to the squad.
"First and foremost, my focus is on helping Glasgow Warriors win tomorrow's massive Heineken Cup match in Bath, but I'm really looking forward to joining up with the rest of the Scotland squad, putting in a lot of hard work and challenging for a place."
Scotland head coach Andy Robinson said: "It's a blow for Fraser but I've always stressed the importance of performing on Scotland A duty and consistently for your club.
"Tom has done this and deserves this chance."
Ryder, the 6ft 5in, 18st 6lb lock, who qualifies for Scotland through his Glaswegian father, Paul, joined Glasgow Warriors from English Premiership outfit, Saracens, on a two-month loan in August 2010 and impressed to the extent where he was invited to make the arrangement permanent.
Ryder has since become a key member of the Glasgow front eight, making 29 appearances in the RaboDirect PRO12 and a further six in the Heineken Cup.
He helped Scotland A achieve a 100 per cent record in season 2010/11, impressing in the wins over USA Eagles and Italy A.
A mainstay throughout England's international age-grades from under-16 up, Tom moved to Saracens from the youth set-up at Leicester Tigers in 2005 aged 20, and made 95 appearances for the club.
Ryder spent the opening three months of the 2008/9 season playing for Taranaki in the Air New Zealand Cup.
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