St Helens (12) 38
Tries: Gardner, Gidley, Meli, Roby 2, Wilkin 2Goals: Long 5
Leeds (4) 10
Tries: McGuire, Ellis Goals: Sinfield
Roby ran rampant in the second half at Knowsley Road
St Helens booked their place in the Grand Final with a brilliant seven-try performance against Leeds.
Ade Gardner gave the home side the lead on seven minutes, but Danny McGuire levelled with a 16th-minute score.
Matt Gidley regained the lead for Saints, who added five second-half scores when James Roby touched down twice, along with Francis Meli.
Two tries from Jon Wilkin further compounded the visitors' misery, while Gareth Ellis scored a consolation try.
It was yet another all-encompassing, dominant performance from Saints on the night when captain Paul Sculthorpe announced his retirement from the game.
The Super League table-toppers had only been beaten once at Knowsley Road all season - a 14-10 defeat by Leeds back in March.
But any ambitions of a repeat performance were soon banished as Saints took control of the game from the early stages.
A brilliant sweeping, passing move from inside Saints' own half enabled Meli to burst free on seven minutes, only for the winger to be hauled down on the 10m line.
But the ball was rapidly recycled and a six-pass move saw Gardner touch down for his 27th try of the season.
But Leeds regained parity on 16 minutes when Gardner failed to gather a hanging kick from Kevin Sinfield on the 20m line, which was touched down by McGuire, although video referee Ian Smith had to verify that Carl Ablett had not knocked on in the build-up to the score.
Gidley soon put the home side back in front when he latched on to a neat pass from dummy-half Keiron Cunningham to bundle over from five yards in the corner, converted by Sean Long.
And Saints extended their lead on the half hour when scrum-half Long added his second kick of the evening after Leeds were adjudged to have stolen the ball in the tackle five metres out from their tryline.
Despite their eight-point deficit, the Rhinos created a sustained spell of pressure on the brink of half time, but Jordan Tansey spurned a gilt-edged chance to bring Leeds to within two points of regular season champions.
The replacement could not cling on to Nick Scruton's reverse offload two metres from the Saints tryline, much to the 22-year-old's frustrations.
Saints soon extended their lead to 12 points courtesy of a moment of brilliance from Man of Steel candidate Leon Pryce on 47 minutes.
The stand-off shrugged off four defenders inside his own half before bursting into open space and spreading the ball for Willie Talau, whose inside pass found Meli in the corner for a simple score.
And before Leeds had time to regroup, Roby further extended Saints' lead on 53 minutes with a short-range try, courtesy of two bruising bursts from prop Nick Fozzard.
Rampant, Saints compounded Leeds' misery when Cunningham's dummy-half run fed the 2007 Man of Steel for his second score four minutes later.
With little left to give, Leeds then gifted Wilkin his first score when he intercepted Sinfield's pass to run 30m to score under the posts.
Although Gareth Ellis barged over for a consolation try for Leeds, Wilkin finished off the rout when he latched on to Long's precise grubber kick on 78 minutes.
St Helens: Wellens, Gardner, Gidley, Talau, Meli, Pryce, Long, Graham, Cunningham, Hargreaves, Gilmour, Wilkin, Flannery.
Replacements: Fozzard, Roby, Clough, Fa'asavalu.
Leeds: Webb, Smith, Ablett, Senior, Donald, McGuire, Burrow, Leuluai, Diskin, Peacock, Jones-Buchanan, Ellis, Sinfield.
Replacements: Lauitiiti, Bailey, Scruton, Tansey.
Referee: A Klein (London)