Sunderland are set to open the Premier Concourse for just the second time this season on Saturday.
Exceptional demand for tickets means that an attendance of almost 40,000 is already confirmed for the League One clash.
It will be the second biggest attendance of the season, with 46,039 watching the 1-0 win over Bradford City on Boxing Day.
Executive Director Charlie Methven said: "Tomorrow is a fantastic opportunity for the SAFC community to come together, with over 40,000 supporters expected to be at the Stadium of Light for a massive game between two grand old clubs.
“We have blown the average attendance record for the third tier out of the water, and to have potentially clocked up two 40,000-plus attendances within one season tells you everything you need to know about our magnificent support.”
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The crowd will be boosted by a strong support from the South Coast, with over 3,000 Portsmouth fans making the journey.
There will be no cash turnstiles on the day and supporters are advised that they must have a previous purchase history with the club.
The game is likely to have a major impact on the push for automatic promotion.
Pompey have a two-point lead heading into the final week of the regular season.
The Black Cats will be eager to secure three points and hope that Barnsley slip up in one of their final two games of the campaign.
Daniel Stendel's side host Blackpool on Saturday before travelling to Bristol Rovers next weekend.