Hayden follows Svetoslav Todorov to Pompey and of course reacquaints himself with Harry Redknapp at Fratton Park.
With pen being pit to paper late on Tuesday evening, the transfer was a somewhat less drawn out affair than when he joined the Hammers from Sanfrecce Hiroshima in August 2000.
Work permit problems meant that he went through his first season encumbered by red tape, eventually joining Mechelen on loan to get match experience.
This season he was thwarted in his attempts to break into the team by the Christian Dailly/Tomas Repka partnership, and has started just four Premiership games for West Ham.
Although he has dropped out of a regular spot in the Australia squad, Hayden, who will be 25 in June, will have no permit restrictions as he is now married to a French national.
A fee is involved, and Paul says of the transfer:
"We wish Hayden all the best at his new club and hope this move will resurrect his career."
A report that said, however, that Hayden asked for a transfer is incorrect.