Leeds (6) 16
Tries: Hall, Hardaker, McShane Goals: Sinfield 2
St Helens (6) 30
Tries: Foster 2, Shenton, Meli, Wellens
Goals: Foster 5
Foster grabbed 18 points in St Helens' voctory at Headingly
Jamie Foster scored two tries and kicked five penalties as St Helens produced a thrilling second-half performance to overcome Leeds Rhinos.
Foster put the visitors ahead before Ryan Hall went over to get Rhinos back on level terms at half-time.
Debutant Zak Hardaker put Leeds in front for the first time, but Michael Shenton then went over for St Helens.
Paul McShane forced his way over for Rhinos before Francis Meli, Paul Wellens and Foster secured Saints' win.
Although missing the suspended Kyle Eastmond, St Helens' younger players led the way as they recorded their third victory in this season's Super League against a Rhinos side who have only won once at home.
Foster, 20, began his 18-point haul midway through the first half when fellow youngster Jonny Lomax, Wellens and Meli combined to put him over before he converted his try from the touchline.
England winger Hall had Leeds level at the break to leave the sides tied at 6-6 after Kevin Sinfield and Carl Ablett had moved the ball wide.
Teenager Hardaker, who was recalled from a loan spell at Featherstone last week, then managed to juggle the ball without it touching the ground to give Rhinos an early second-half lead before Shenton beat Hall to Gaskell's high kick to put Saints back in front.
Leeds then fought back as the visitors allowed replacement hooker Paul McShane to force his way over from dummy half.
But the lead then changed hands for a fourth and final time as Meli latched on to Scott Moore's pass to break through the Leeds defence before Foster kicked Saints to an 18-16 lead.
Joint-captain Wellens then collected Gaskell's pass to give St Helens a comfortable advantage before Foster made sure of the win with some impressive handling three minutes from time.
Leeds coach Brian McDermott:
"I thought we were in charge of that game. There was a point in that second half when we should have been stronger and we weren't.
"The momentum fizzled out and there is no real reason for it. We don't recognise when the game is there to be closed down.
"I thought generally we took another couple of steps forward defensively. We showed outstanding commitment to each other in the first half but, on the flipside, conceded some soft ones which we're going to learn from.
"It's disappointing that we conceded the tries like we did."
St Helens coach Royce Simmons:
"The attitude of the players all week was very positive. They brought a lot of energy to training.
"We didn't let last week get us down, they were very professional all week.
"Our ball control was very good and we were patient. The young boys persisted in what they thought was right and that in the end got us over the line."
Leeds: Webb; Hardaker, Watkins, Senior, Hall; Sinfield, Burrow; Jones-Buchanan, Buderus, Kirke, Hauraki, Lauitiiti, Ablett.
Replacements: Cross, McShane, Burgess, Clarkson.
St Helens: Wellens; Gardner, Shenton, Meli, Foster; Gaskell, Lomax; Perry, Roby, Graham, Clough, Flannery, Puletua.
Replacements: Wheeler, Moore, Ashurst , Magennis.
Ref: Richard Silverwood (Dewsbury).