The 17-year-old will enter a booming academy set-up at Melwood but is already targeting a first team breakthrough.
"Right now, I want to make sure that wherever I go I am in the right place where there is a balance between training and playing high-level matches. And I want to ensure I can continue my education."
Pelosi can play either as a centre back or a defensive midfielder and has played, 28 times for the US Under-17s, scoring eight goals.
He captained America to the round of 16 in this summer's Under-17 World Cup.
Academy director Frank McParland told liverpoolfc.tv: "We're determined to identify the best possible talent globally and I'm delighted we've been able to bring Marc in to the club."