The Hartlepool-born 21-year-old will be eligible to play in Pompey’s next game – which is against Hartlepool United at Fratton Park this Saturday.
Reed will sign for an initial month-long spell, although his stay could be extended to the end of the season if it goes well.
He has been with Sunderland’s Academy since the age of eight.
He has been on Sunderland’s bench several times but never featured in a first-team game.
He has made 12 appearances during a loan spell at Brentford between February and May 2011.
He was then loaned out to both Bradford and Leyton Orient in 2011-12, playing 15 games in total.