Rovers’ official website said winger Marcus had joined the north-west side on a free transfer from Swedish club Halmstad on a two-and-a-half-year deal until 2014.
The 23-year-old is now awaiting international clerarance from the Swedish FA and FIFA.
His signing comes six years after central defender Martin’s transfer deadline day arrival at Ewood Park.
The brothers started their careers at local side Hogaborgs BK and had a trial with Blackburn at the end of 2005.
But while Martin signed a contract under then manager Mark Hughes, Marcus returned to Sweden and eventually signed for Halmstad in 2008, going on to score 17 goals in 139 appearances.
Marcus Olsson is the second player signed by Blackburn boss Steve Kean in the transfer window following the recent loan deal for Bordeaux striker Anthony Modeste.
Rovers are currently in the relegation zone, one point shy of safety, ahead of Wednesday’s league match at home to Newcastle.