The 21-year-old prop or back row has joined on a dual registration and will go straight into the side to play South Wales on Sunday.
“The fact that he is joining us on a dual registration and not a loan deal shows how much St Helens think of him,” said Swinton director of rugby Paul Kidd.
“He is a young player of quality who has come through the St Helens system and with our current injury situation will provide just the pick-me-up we need.”
Thompson, who has been captain of the Saints reserve side, played alongside Swinton hooker Karl Ashall with the Merseyside club last season.
Fullback Richie Hawkyard has faced a setback with his hamstring injury while Joe Fitzpatrick has been in hospital with a stomach complaint.
“We are still struggling a little bit and we won’t know where we are up to until the last minute,’ said Steve McCormack.
“Tommy Gallagher’s knee problem will keep him out and Richie Hawkyard now needs to see a consultant over his hamstring.”
McCormack predicts a tough test from the Welshmen who have picked up one win from four games.
“They played well against Workington last week but did not get a result,” he said.
“They are showing signs of improvement and are a young side with bags of pace.
“They are very competitive and it’s a tough trip for us; we will have to be fully focussed.”