Fulton, 19, is a Scotland Under-21 international who has yet to play first-team football for Liverpool.
He was named as an unused substitute for the Reds in two League Cup ties against Carlisle and Bournemouth earlier this season.
Fulton is expected to provide competition for Portsmouth goalkeepers Brian Murphy and Paul Jones.
Meanwhile, Pompey defender Ben Davies has revealed he is close to signing a new deal with the club.
"I'm enjoying my football and whenever that's happened in my career, I tend to get the best out of myself," the 34-year-old told BBC Radio Solent.
"Being surrounded by good players and a good manager always helps as well."
Davies joined Portsmouth in the summer after being released by Sheffield United. He has been ever-present in League Two since then, scoring one goal.
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Portsmouth have signed teenage Liverpool goalkeeper Ryan Fulton on a youth loan until the end of the season.