Forest are willing to send Henderson out on loan after sealing a loan move for Watford goalkeeper Costel Pantilimon.
Henderson has made just one appearance for Forest this season with Jordan Smith establishing himself as first-choice keeper at the City Ground.
The 29-year-old has been unable to hold down a spot in the Forest side since recovering from the ruptured Achilles he suffered almost a year ago.
Henderson is keen to go and out get regular first-team football and Portsmouth are ready to offer him that opportunity.
The Irishman has had a spell at Portsmouth earlier in his career, making 25 appearances during his time at Fratton Park.
Henderson is one of several players to exit Forest on deadline day with Aitor Karanka reshaping his squad with an influx of arrivals and departures before Wednesday's transfer deadline.