With a magnificent display that served as a glorious reminder of their halcyon days, Huddersfield came up with their best 80 minutes of the season to book a trip to Twickenham.
Up to a fortnight ago, Huddersfield were battling against relegation but they made a mockery of the engage Super League standings to run last year�s runners-up ragged in a memorable semi-final at Odsal.
Jon Sharp�s men were inspired brilliantly by Australian hooker Brad Drew, the man of the match, while second rower Chris Nero and winger Stuart Donlan grabbed two tries apiece and centre Michael De Vere finished with 14 points.
The victory was especially sweet for evergreen scrum-half Robbie Paul, who will relish a sixth Challenge Cup final appearance.
Leeds were actually on course for their fifth final in eight years when they scored the game�s first try but the Rhinos looked strangely out of sorts throughout and a third successive defeat will have the alarm bells ringing at Headingley.
Tony Smith�s men badly missed the penetrative running of Danny McGuire, who failed to recover from a shin injury, while Huddersfield kept the shackles on livewire scrum-half Rob Burrow.
The Giants, humiliated 46-6 in their last semi-final by St Helens two years ago, looked more energised from the start, as they kept the Rhinos pinned inside their own half with some aggressive defence.
Two early knock-ons by Great Britain forward Jamie Peacock, back on his Odsal stamping ground, and a mixed performance by full-back Lee Smith summed up Leeds� shoddy first-half display.
Smith put his side under enormough pressure by twice fumbling high kicks from Huddersfield captain Chris Thorman, which led directly to one try and also a disallowed effort.
Giants winger Stuart Donlan had a score ruled out by video referee David Campbell, who decided that Paul Smith gathered a loose ball from an offside position, and was then hauled into touch by Burrow after being freed by the impressive Chris Nero.
It was very much against the run of play when Leeds opened the scoring on 15 minutes with centre Chev Walker working his winger Scott Donald over at the corner.
When Kevin Sinfield�s conversion went in off an upright, it might have been a sign that it was not going to Huddersfield�s day but their response was swift and decisive.
Hugely-impressive full-back Paul Reilly made the running for Nero to cross on 20 minutes and five minutes later Donlan took advantage of a moment�s hesitation to win the race to Thorman�s grubber kick to the corner.
Michael De Vere improved both tries to put the Giants 12-6 ahead and they almost scored again when big Eorl Crabtree was held up on his back over the line.
It was 30 minutes before Huddersfield came up with their first handling error but it was to prove a costly mistake, with Leeds setting up the position for substitute Willie Poching to bounce off the challenge of Thorman to grab a try.
Sinfield�s second goal tied the scores at the break but Huddersfield were undaunted and went back in front 12 minutes into the second half.
Substitute Steve Snitch re-gathered possession from Drew�s kick over the Leeds defence and Paul was on hand to send Nero over by the posts for his second try on 52 minutes.
And it got even better for the underdogs seven minutes later when Donlan, too, claimed his second touchdown, this time after a jinking run from Paul and a clever long pass from Thorman.
De Vere was off target for the first time but Huddersfield had a 10-point cushion and the Australian drove another nail into Leeds� coffin with two penalties as the Giants tightened their grip.
Leeds: (12) 12 Tries: Donald, Poching Goals: Sinfield 2
Huddersfield: (12) 30 Tries: Donlan (2), Nero (2), De Vere Goals: De Vere (5)
Huddersfield: Reilly, Aspinwall, Wild, De Vere, Donlan, Thorman, Paul, Jackson, Drew, Gannon, Nero, Smith, Jones. Replacements: Crabtree, Snitch, Mason, Raleigh.
Leeds: Smith, Donald, Walker, Senior, Gibson, Sinfield, Burrow, Jones-Buchanan, Diskin, Scruton, Lauitiiti, Peacock, Ellis. Replacements: Kirke, Millard, Poching, Bailey.
Referee: R Silverwood (Dewsbury)