The 23-year-old has made the switch after being informed by Liverpool manager Brendan Rodgers that he did not feature in his immediate plans at Anfield.
Discussing his switch, the former Newcastle United centre-forward admitted that West Ham manager Sam Allardyce played a big role in tempting him to Upton Park.
"It's great to be here at West Ham and all signed up. I just can't wait to get started now," Carroll said. "I want to be playing games and obviously hoping to score some goals. I know the manager well and some of the lads, so it is nice for me to come to a place where I know people.
"I'm hoping to add a lot of goals and create a few chances. I know Sam from Newcastle and he was a massive reason for me coming. When I worked with him at Newcastle it was great so it was a no-brainer really."
Carroll could make his debut during Saturday's home encounter against Fulham.