Saints were not at their scintillating best against the East Yorkshire side; but it would be unfair to discredit a Hull side that will be confident of making it to Old Trafford with a win over Bradford next week.
After a tight opening Ade Gardner strolled through the static Hull defence to give the home side a lead.
However the black and whites kept themselves in the game and a try from Kirk Yeaman allowed them to draw level.
The two sides exchanged penalties and the clash was set up for a grand stand finish until Francis Meli’s 61st minute touch down settled the game in Saints’ favour.
St Helens boss Daniel Anderson praised the quality of football served up by both sides, saying: "It was very exciting and the nerves are still jumping.
"I was very impressed by both teams and there was some absolutely fantastic defence on display. It was a wonderful exhibition of semi-final football.
"Defence has been the cornerstone of everything we've done this season and it's what won us this match."
Hull coach Peter Sharp credited his side with an impressive performance, admitting: "It was a tremendous effort, but we came up a little bit short.
"It was a tough match, very hard and very skilful and we will take something from it. We'll regroup and be ready for next week.
"I thought we didn't get the rub of the green all night. There were some dubious decisions to say the least."