The highly-rated 23-year-old has attracted plenty of interest from clubs across Europe during his time at Nottingham.
However Gloucester have convinced Kalamafoni, who appeared in all Tonga's games at the World Cup, to make the switch to the west country.
Gloucester coach Bryan Redpath said: "I'm delighted that Sione has agreed to join us as he's proved himself to be one of the stand out players in the Championship and there was a lot of interest in him.
"He's a big athletic back rower who is comfortable running with ball in hand but also a physical presence in defence.
"He's proven himself to be a capable performer at international level and we think he's got what it takes to make a real impression in the Aviva Premiership."
Although delighted to be making the step up to the Premiership, Kalamafoni admitted it would be hard to leave Nottingham after two seasons at Meadow Lane.
"It was hard and very easy at the same time," Kalamafoni said.
"I'm sad to leave the first professional club that I joined, and Nottingham has been very good helping me on and off the pitch. And then it is very easy wanting to go to a Premiership club like Gloucester."
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