Sunderland fans travelling to Fratton Park are being reassured that "enchanced" security measures will be in place after threatening messages were posted on social media.
The Black Cats will face Pompey away in the second leg of their League One play-off semi-final at 7.45pm today.
The home leg took place at the Stadium of Light on Saturday, where Sunderland secured a 1-0 win.
Ahead of the match, messages are believed to have been shared on Twitter on Wednesday night making threats towards football fans attending the game.
It is thought they have since been deleted.
Now, both Portsmouth Football Club and Hampshire Police have moved to step up security, and are working in partnership to ensure safety at the fixture.
The club issued a statement regarding the threats, reassuring fans about "enhanced" security measures at tonight's match and urging ticket holders to arrive in plenty of time to undergo checks.
It said: "Portsmouth Football Club are aware of a message that has been posted on social media and are working with police on this matter.
"The club would like to reassure fans that there will be enhanced security checks in place for Thursday’s play-off match against Sunderland at Fratton Park (7.45pm kick-off).
"Due to these checks, all supporters with tickets are advised to arrive in plenty of time to make sure they are inside the stadium for the start of the game.
"Tickets for the match – which is being televised live on Sky Sports – have sold out."
In a statement posted on social media, Hampshire Constabulary also confirmed the force's action in looking into the posts.
It said: "We are aware of threats made on social media regarding the football match between Sunderland and Portsmouth taking place in Portsmouth tomorrow, (Thursday).
"We are looking into these posts and want to assure the public that a sufficient policing response will be in place for the match tomorrow and that we are working alongside Portsmouth FC."