St Helens (6) 12
Tries: Puletua Cunningham Goals: Lomax, Eastmond
Catalans Dragons (10) 24
Tries: Pelo, McGuire, Elima, Greenshields Goals: Bosc 4
Greenshields scored the Dragons' fourth try on 67 minutes
Catalans Dragons produced a surprise win over St Helens to make the play-offs for the second successive year and bring Bradford's season to an end.
Dimitri Pelo's try gave the visitors an early lead which they reclaimed before half-time as hooker Casey McGuire barrelled over.
Jean-Philippe Baile and full-back Clint Greenshields then crossed as the Saints defence melted away in the second half.
A first-ever Knowsley Road victory was secure despite Keiron Cunningham's try.
Although they started the night in possession of the season's final available play-off place, the Catalans' grip on eighth was under pressure.
An in-form Bradford team travelled to Hull knowing that if they could manage a better result than the Perpignan side, it would be their own season extended to at least another match.
But winger Pelo combined with Baile after just four minutes to put his injury-blighted side ahead and make light of the loss of six first-team regulars.
The hosts, who again looked short of form after a less-then-impressive 12-10 win over Huddersfield in their last home outing, hit back through prop Tony Puletua's burst from Sean Long's pop.
But Kevin Walters' side took a deserved lead into the break with the livewire McGuire barging over from short range and Thomas Bosc adding the extras.
Ade Gardener fumbled an excellent chance to wrestle the match back St Helens' way as the ball was spread wide after an electric break from Kyle Eastmond.
Baile drove through a mass of defenders on 56 minutes after Francis Meli's knock-on and a pinpoint conversion from Bosc further stretched the Dragons' lead.
Greenshields carved through the remnants of the St Helens cover to touch down 11 minutes later and effectively seal the win, although Saints, who finish runners-up in the league, earned some scant consolation through Cunningham's score.
St Helens will play either Hull KR or Huddersfield in a qualifying play-off while Catalans will play either Wigan or Wakefield in an elimination match.
St Helens coach Mick Potter:
"It wasn't our best performance. We came up with far too many errors and contributed to our own downfall.
"We looked down on our line speed in defence and our execution in attack was well below par but it's fixable. We have to build for next week. We start afresh. It's a new competition now.
"They played with a lot of vigour and executed some really good plays. I thought Casey McGuire, Adam Mogg and Thomas Bosc were very good. I'm pleased for them but I hate losing."
Catalans Dragons coach Kevin Walters:
"If we put it together, we can get results like this.
"We understand that St Helens had less to play for than us and were down a bit but there was a lot of pressure on our players from a lot of different sources and I'm delighted for them.
"I was particularly happy with our defence because it's been lacking at times but there was a lot more sting in our tackles tonight.
"I think this will give them a lot of confidence because we've not had too many good wins this year."
St Helens: Lomax, Gardner, Gidley, Gilmour, Meli, Pryce, Long, Hargreaves, Cunningham, Puletua, Flannery, Wilkin, Clough.
Replacements: Roby, Eastmond, Fa'asavalu, Emmitt.
Catalans Dragons: Greenshields, Duport, Baile, Raguin, Pelo, Mogg, Bosc, Guisset, McGuire, Ryles, Elima, Gossard, Bird.
Replacements: Ferriol, Touxagas, Casty, Martins.
Referee: B Thaler (Wakefield)