Hirst joined the Foxes in 2019 from Belgian sister club OH Leuven and spent last season on loan at Rotherham United.
Speaking about the signing of Hirst, Portsmouth manager Danny Cowley said: "George is still a really young boy, but it feels as though he's been around forever.
"He played for Leicester's feeder club in Belgium before signing for them and we've followed his career closely.
"He didn't have it all his own way at Rotherham last season. He didn't play as much as he would have liked and it probably wasn't the right fit for either party.
"Sometimes that happens in football, but we like to sign players who have got a point to prove and we see a real hunger and desire to succeed in George.
"When you don't have everything your own way, it can really motivate you. We certainly believe in his abilities."
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