The shot-stopper has dropped down to League One to sign for Danny Cowley’s men on a season-long loan deal and will be looking to clock up regular game time at Fratton Park.
However, West Brom only allowed the goalkeeper to head to the south coast because of a special recall clause in the agreement.
The loan terms give West Brom the option to instantly recall Griffiths, within 24 hours, if either David Button or Alex Palmer pick up an injury.
Baggies boss Steve Bruce does not want to be left short of cover and Portsmouth had to agree to the clause in order for the loan deal to go through, with Albion custodian Ted Cann not having played five professional games.
The goalkeeper was on loan at Lincoln City last season and made 33 appearances in League One.
He only managed to keep two clean sheets and will be looking to vastly improve on that number with Portsmouth in the coming campaign.
Cowley will be looking for Portsmouth to push for promotion to the Championship this season and will hope Griffiths can get up to speed quickly.
