Gilmour, 31, moved to Knowsley Road from Bradford in 2003 having started his career with Wigan. He has made more than 300 appearances in Super League and will join up with the Giants for the 2010 season.
The Scotland international said: “I am absolutely delighted to be coming to Huddersfield and I am already excited about next year. Although I had a number of offers on the table, Huddersfield were my first choice and it’s fantastic we’ve been able to sort a deal out. There is no doubt this is a club on the up and the Giants can now boast a squad genuinely capable of challenging for silverware.
“Having spoken to Nathan Brown and listened to what people have had to say about him I know I can continue to improve which I believe is very important. Even though I won’t be joining until next season I already know this is the place where I want to see out the rest of my career.”
Huddersfield head coach Nathan Brown said: “This is good news for the Club. Lee Gilmour is a winning player and he has won at every club he’s been at in Wigan, Bradford and St Helens. He is versatile and can play in the back row and in the centres. Lee has got good leg speed and can beat people; he is a good trainer and is very professional. We believe he is a player that can help improve our squad.”
Meanwhile, Saints chairman Eamonn McManus has paid tribute to the former Great Britain international.
He said: “Gilly has had a great six seasons with us during which he has won every honour possible.
“He has been a great player as well as a committed professional and club man and has been a massive contributor to our success.
“The salary cap means that we cannot keep everyone that we wish. We will have an abundance of second rowers next year with the emergence of new young talent within the club, along with the signing of 22-year-old New Zealand international Sia Soliola, who will play back row/centre.
“These decisions are terribly difficult to make but we must evolve the squad for the future as well as enjoying it at the present. We wish Gilly every luck for the future and I’m sure that the fans will give him all their support for the rest of the season as well as a heart-felt send off at the end of it.”