Billy Davies was keen to add some cover for Callum Davidson and is delighted that Curtis has agreed to the switch.
“We are very happy John has agreed to join us,” Davies told the club’s official website.
“With Callum Davidson still needing two or three weeks training and Tyrone Mears still injured, we were short of full-back cover and John can play on both sides.
“He has played at a very high level consistently throughout his career and he has seven games with us and hopefully he can play an important role for us during that period.”
Twenty-six-year-old Curtis began his career with Manchester United but made just four Premiership starts before making a £1.5million move to Blackburn Rovers in June 2000.
He was an ever present during Rovers’ promotion season in 2000-01, but then made just 15 Premiership appearances in the next two years.
A move to Leicester City followed at the beginning of last season, but he failed to settle and moved on to Pompey during the January transfer window, where he has since made just five Premiership starts.