The fixture is in addition to the scheduled game between the Jacksonville Jaguars and San Francisco 49ers on October 27 next year.
The NFL has hosted a regular-season game at Wembley in each year since 2007, but this will mark the first time there has been more than one in a single season, an idea which has been under discussion for some time.
The decision was approved by NFL owners meeting today in Chicago.
"Since we started playing regular-season games in London five years ago, we have heard very clearly from our UK fans - they want more football," said NFL commissioner Roger Goodell.
"We are excited to play two games in London and take this next step in the growth of our game. We believe that more football will lead to more fans."
The Vikings will give up a home game in order to make the trip, with this being a one-year commitment from the Minneapolis-based franchise. The Jaguars have committed to hosting games in London each year from 2013 to 2016 in a bid to establish a foothold in Britain.
"This is a unique opportunity for the Vikings organisation to highlight our brand on an international level," said Vikings owner and president Mark Wilf.
In the Steelers, London will get to see one of the NFL's most storied franchises, winners of a record six Super Bowls.
"We are thrilled that our players and Steelers fans around the world will be able to enjoy the atmosphere at historic Wembley Stadium," said Steelers president Art Rooney II.
This year, the St Louis Rams are to play the New England Patriots on October 28.