The rain ensured a low-scoring game with handling errors throughout.
The win keeps St Helens in third place, now three points ahead of fourth-placed Huddersfield with three games left of the regular season.
The action came to a stop after two minutes after a clash of heads between James Graham and Luke Robinson – Robinson stayed down for a few minutes but was eventually able to carry on.
Chances were few and far between: Huddersfield had the best chance of the opening quarter, a kick from Danny Brough knocked back but Lee Gilmour was unable to take possession.
Instead it was the Saints who opened the scoring, Leon Pryce finding his way through the Giants defence and racing over to score, Jamie Foster converting.
St Helens’ lead was extended after the half-time interval, Pryce finding Wellens who squeezed between two tacklers to touch down, Foster’s goal stretching the advantage to 12-0.
Huddersfield saw Luke O’Donnell was stretchered off with a knee injury, before a third Saints try when Jon Wilkin found Louis MacCarthy-Scrasbrook, who weaved his way over the line, Foster converting once more.
The Giants finally crossed when Brough’s kick to the in-goal area was brilliantly knocked back by Kevin Brown and touched down, Simon Grix’s goal reducing the gap to 18-6.
But even the sin-binning of Paul Clough for dissent couldn’t help Huddersfield, and after Brough was sidelined through injury Foster sealed St Helens’ win with a drop goal six minutes from time.