Kevin Brown and Michael Lawrence celebrate the Giants' win
Huddersfield (16) 28
Tries: Griffin 2, Lolesi, Whaturia, Jensen
Goals: Thorman 4
St Helens (20) 26
Tries: Wellens, Flannery, Pryce, Roby
Goals: Long 5
St Helens fell to their third defeat of the year as Huddersfield recovered from a 14-0 deficit to record one of the biggest shocks of the season.
Saints made a blistering start with Paul Wellens and Chris Flannery scoring early on, but they tired in the second half as Huddersfield fought back.
Prop Darrell Griffin returned from a ban to score two of his side's tries.
Rod Jensen's late try then put the Giants ahead but Francis Meli wasted the chance to steal it for Saints.
St Helens came into the match with the memory of their 46-10 demolition of Wigan still fresh in their minds, and they hit the ground running at the Galpharm Stadium.
Half-backs Leon Pryce and Sean Long ran the show for the visitors, while James Roby's runs from dummy-half caused havoc for Huddersfield.
Long's well-placed kick allowed Wellens to touch down after only seven minutes, while Flannery finished off a half-break by Pryce 10 minutes later.
A Long penalty made it 14-0 and it looked ominous for the Giants, who were on a three-match losing run.
But Griffin provided some much-needed inspiration as he went over for their first try after 24 minutes.
Four minutes later, Jamahl Lolesi took Stuart Jones's pass and cut inside for his side's second try, but Pryce latched onto a Long kick to keep Saints in front.
Paul Whatuira touched down Chris Thorman's kick to the line for a controversial score on the brink of half-time, with the actual touchdown coming after the hooter had sounded.
Saints prop James Graham was on the losing side at the Galpharm
Huddersfield went in front for the first time two minutes into the second half after a comedy of errors from St Helens, with Long's penalty failing to find touch and Meli losing the ball on the first tackle.
Griffin took advantage to score his second try and Thorman's conversion made it 22-10 to the Giants.
Roby forced his way over from dummy-half to restore St Helens' lead, but Lolesi released Jensen to score his team's fifth try.
Saints could have won it at the death, but Meli fumbled after Jensen had missed Jon Wilkin's high kick to the corner.
Huddersfield coach Jon Sharp:
"I was really pleased with the response. I don't think we've been far off in the last few weeks.
"The issue I had was discipline - but I thought we smartened our act up.
"Our four new faces were brilliant - especially young Michael Lawrence, who is only 17. The importance of fresh people over the Easter period cannot be over-estimated.
"I knew that if we were just one score away from them by half-time, we'd be in with a chance.
"I thought the bench was the difference between the two teams. We had some quality in there and I thought we wore them down."
St Helens coach Daniel Anderson on the performance of referee Ian Smith:
"We've had him for four of eight games, and I've had my dose of him. I wonder if he's up to this standard.
"I'm bemused by his performance in general. We didn't get a single one of the 50-50 calls.
"But we defended nowhere near the level required. We allowed some extremely soft tries.
"Darrell Griffin's try early in the second half was typical of how we've played this year.
"We do very well until late in the tackle count, and he strolls through three players. It was a difficult afternoon for us."
Huddersfield: Thorman, Lawrence, Whaturia, Lolesi, Jensen, Brown, Robinson, Mason, Gatis, Skandalis, Snitch, Crabtree, Jones.
Replacements: Raleigh, Jackson, Griffin, Hudson.
St Helens: Wellens, Gardner, Gidley, Talau, Meli, Pryce, Long, Fa'asavalu, Roby, Graham, Gilmour, Flannery, Wilkin.
Replacements: Bennett, Clough, Eastmond, Frodsham.
Referee: I Smith (Oldham)