The 30-year-old Australian-born forward represented Scotland at the 2013 Rugby League World Cup.
He holds the record for the most consecutive appearances in the NRL with 215 between 2006 and 2014.
"The club has a great profile and stature in the game and I hope to be able to contribute to developing that wonderful culture," he said.
Douglas will join St Helens ahead of the 2017 season.
"I am told Saints have the best fans in the game and I can't wait to engage with them. I'm really looking forward to this next chapter in my life," he added.

Christina Haycock, 49, of Mold, Flintshire, caused council offices and courts to be evacuated, Caernarfon Crown Court was told.
She admitted making a bomb threat to Flintshire council on 22 January.
Defence counsel Jade Tufail said she wanted an apology "from those who were supposed to protect her at the time".
The court heard Haycock, a criminology and criminal justice graduate who had been in care from the age of three, told a council phone operator: "Shire Hall has got a bomb in it so I would tell them to get out."
Prosecutor Elen Owen said the call caused council staff, judges, magistrates and other court users in Mold to be evacuated with an estimated cost of tens of thousands of pounds.
Judge Philip Harris-Jenkins said: "In the circumstances of this case, because of the gravity of the offence, the disruption it caused, I am afraid I am not going to be in a position to suspend the sentence."

The crash was on the A96 Forres bypass.
It took place early on Tuesday evening.
The man's injuries are not thought to be life-threatening.

In Swansea, the party added one seat to hold 48 out of 72 seats, while Plaid Cymru made inroads picking up seven new seats in neighbouring NPT.
Pembrokeshire remains in the hands of independents, while Carmarthenshire has no party in overall control.
Swansea council leader Rob Stewart said Labour's success in the city was because it had offered hope.
"We came into this election on a very positive manifesto, offering people hope, aspiration, a better city, a better life," he said.
"We'd just delivered the City Deal for Swansea, the city centre is being rejuvenated, we're after the Tidal Lagoon - we're hoping to land that very shortly, and we're bidding to be the city of culture in 2021.
"We've demonstrated we can deliver for the people of Swansea and we're standing on an even more ambitious programme for the next five years."
In Swansea, independents took nine seats, Conservatives eight and Liberal Democrats seven.
Meanwhile in Neath Port Talbot, Plaid Cymru made inroads picking up seven new seats.
In Pembrokeshire, former deputy council leader Rob Lewis lost his seat in Martletwy to Di Clements of the Conservatives.
Independent candidates won 34 seats in the county, Conservative had 12, Labour seven, Plaid Cymru six and the Lib Democrats gained just one.
In Carmarthenshire, Plaid Cymru won the most seats with 36, while Labour gained 22 and the independents 16.

Dean Saunders, 25, died at HMP Chelmsford, Essex, in January 2016.
He had been arrested after he stabbed his father during a paranoid episode.
Kathy Goodchild, clinical team leader at the prison, told the inquest into his death that psychiatrists were not always present during reviews of care given to vulnerable prisoners.
LIVE: For more on this and other Essex stories
She told how when she was involved in a review of Mr Saunders' care on 20 December 2015 he eventually told her he did not intend to harm himself.
However, she said better psychiatric cover was needed and there was "not always a psychiatrist on call" for things like approving medication.
She wrote on 20 December that Mr Saunders should be seen by a psychiatrist the next day but that did not take place.
"Any review," she said, "should be done by people who actually have got the information."
She said Mr Saunders' family - who have strongly criticised the prison service for not watching him more closely - had raised concerns about him harming himself with her.
However, she said: "I can't put people on constant watch because of families, it has to be on presentation or harm."
The hearing continues.
Super League side St Helens have signed prop Luke Douglas from NRL's Gold Coast Titans on a three-year deal.
[NEXT_CONCEPT]
A woman who made a bomb hoax call because she wanted a council to apologise for her treatment as a child in care has been jailed for a year.
[NEXT_CONCEPT]
A man has suffered serious injuries after his motorbike and a car collided.
[NEXT_CONCEPT]
Labour has increased its majority in Swansea and held on to control of Neath Port Talbot council.
[NEXT_CONCEPT]
Better psychiatric cover is needed at a prison where an inmate was found electrocuted in his cell, an inquest has heard.