Despite leading through Darren Bent's early strike, the Black Cats performed poorly throughout and paid the price for two dreadful defensive errors.
Their FA Cup exit means the only thing they have to play for in the rest of the season is Premier League survival and, on today's evidence, that cannot be taken for granted.
Ironically, the visitors could hardly have wished for a better start as they took the lead with their first meaningful attack in the 15th minute.
The returning Michael Turner flicked on Bolo Zenden's throw, and Darren Bent stole ahead of full-back Anthony Vanden Borre to volley home his 15th goal of the season.
The strike should have steadied the away side's nerves, but they grew increasingly sloppy as a low-quality first half wore on, and could have few complaints when Portsmouth levelled three minutes before the break.
The goal was a horror show from Sunderland's point of view, with John Utaka out-muscling Phil Bardsley before heading Marc Wilson's long ball past a dreadfully out-of-position Craig Gordon.
Yet if that was sloppy from a Black Cats' perspective, Portsmouth's second in the 58th minute was equally as bad.
Younes Kaboul headed Gordon's clearance back into the Sunderland half, and Utaka outpaced both Turner and Paulo Da Silva before slotting a precise finish into the net.
PORTSMOUTH (4-5-1): Begovic; Vanden Borre, Kaboul, Wilson, Hreidarsson; Boateng, Mokoena, Brown (Basinas 9), Diop (Mullins 69), Utaka; Piquionne (Webber 90).
Subs (not used): Ashdown (gk), Sowah.
SUNDERLAND (4-4-2): Gordon; Bardsley (Healy 86), Turner, Da Silva, Richardson; Henderson, Meyler (Reid 73), Cana, Zenden; Bent, Jones (Campbell 81).
Subs (not used): Carson (gk), Malbranque, Murphy, R Noble.