The 33-year-old has been without a club since leaving Cardiff City at the end of last season.
He formed part of the Bluebirds squad which secured promotion to the Premier League, but spent much of the campaign frustrated on the sidelines.
McPhail was restricted to just two appearances in 2012/13, suggesting he will be short of match sharpness.
Wednesday are, however, convinced that they can put his experience to good use.
The Republic of Ireland international has represented Leeds United, Millwall, Nottingham Forest, Barnsley and Cardiff over the course of a distinguished career which has seen him figure in over 400 games.
McPhail has worked with Wednesday boss Dave Jones before, during his time in South Wales, and will be eligible to make his Owls bow against Birmingham City on Saturday if deemed ready for action.