The Black Cats were given an early boost, Dean Whitehead converting a fourth-minute penalty, but Mick McCarthy's side were overwhelmed by a four-goal blast from an impressive Portsmouth.
Man-of-the-match Matthew Taylor scored twice and played a part in the other two goals as the home side crashed to a third successive defeat. And on the evidence of this abject performance Sunderland, without a home win all season, face a desperate battle to avoid an immediate return to the Championship.
Zvonimir Vukic wiped out Sunderland's first-half lead immediately after the break and a double from Taylor rocked the home supporters to the core, but when Dario Silva headed a fourth, the stadium quickly emptied.
Sunderland recalled the six players left out of the midweek Carling Cup defeat by Arsenal but influential midfield pair Julio Arca and Christian Bassila and central defender Gary Breen failed to recover from injury.
Portsmouth included only Andy Griffin of the four players signed from Newcastle United, Andy O'Brien and Lomana LuaLua were out injured and winger Laurent Robert storming out after being named as a substitute.
The Black Cats made a dream start, scoring from the penalty spot after just four minutes. Andy Welsh's cross found Justin Hoyte, whose shot was blocked and when Welsh tried again, Andy Gray's header was punched away by Brian Priske.
Referee Mark Halsey had no hesitation in pointing to the spot but took no further action against the Pompey defender. Whitehead stepped forward to blast the penalty low to goalkeeper Jamie Ashdown's left and just inside the post.
A free-kick from Vukic to the far post was scrambled away from the lurking Silva and then a superb pass from Taylor was just beyond the reach of the Uruguayan.
Sunderland, however, were only inches away from increasing their lead in the 14th minute when a superb volley from Whitehead from the edge of the area beat the despairing dive of Ashdown as the ball whistled past the upright.
A poor clearance from Hoyte was seized upon by Silva but fortunately for the home side goalkeeper Kelvin Davis beat out the close-range shot. Pompey kept up the pressure in search of an equaliser but constant offside decisions, especially on the behalf of Silva, halted their progress.
Griffin was booked for a challenge from behind on Welsh but in a highly-competitive encounter, goal-scoring chances were few and far between, until the visitors went berserk in the second half.
Pompey took just three minutes to find the net after the break. Gregory Vignal's cross was helped on by Taylor for Vukic to hit a crisp left-foot shot across Davis and into the bottom corner.
Pompey were gifted a second in the 59th minute, as another dreadful mix-up between Alan Stubbs and Davis gave Taylor the chance to drive the ball home.
And in the 67th minute the England Under-21 international increased the lead with a bizarre goal. With Silva writhing on the ground seeking treatment and John Viafara trying to come on as a substitute, play was allowed to continue.
Davis seemed oblivious to the fact the game was still in progress and the sleeping keeper was left hopelessly stranded by Taylor's outrageous 40-yard effort.
There was worse to come for a totally demoralised Sunderland when Taylor worked himself to the byline and centred for Silva to send a diving header past Davis to the delight of the Portsmouth fans behind the goal.
Man of the Match: Matthew Taylor.
The sweet left foot of the England Under-21 international completely transformed the game in the second half. The killer third goal from 40 yards completely knocked the stuffing out of the home side.