The deal ends weeks of speculation that has continued to twist and turn. As the final day of the summer transfer window approached, an agreement between the two clubs was struck.
Arriving at Upton Park after a horror spell at Anfield, Carroll is right to focus his immediate attention away from Merseyside.
As the 'Rodgers Revolution' gathers pace, a season away from the club is the best decision for all parties involved.
Here, I look at the five main reasons why Carroll is right to leave. Do you agree? Let me know in the comments section below.
Andy Carroll needs to be on the pitch.
Before he joined Liverpool, the towering striker was told to lose weight (via The Daily Mirror). With his height and fairly stocky build, a lack of game time creates fitness problems. It doesn't matter how intense a training regime is, every player needs to overcome general rustiness by taking to the field.
Last year, Carroll started 30 games in all competitions for Liverpool. He also came off the bench 19 times. For a player who has international ambitions, he needs to be in the original 11 more often.
Even when Carroll performed poorly, Kenny Dalglish always remained faithful in the Englishman's ability. A handful of excellent displays towards the end of the season saw Carroll called up to his country's Euro 2012 squad where he scored a vital goal against Sweden.
Brendan Rodgers doesn't have the same belief in the former Newcastle player.
Although he acknowledged it would be ludicrous to sell Carroll for a snippet of the £35 million paid (via The Express), it was obvious from the moment Rodgers took the reign he wasn't going to use the forward on a regular basis.
The Premier League season is only two games old. Even so, Carroll has managed a disappointing total of 20 minutes on the pitch. While he was named in Liverpool's Europa League squad, he didn't enter the fray once.
At West Ham, Carroll is guaranteed time to make an impression. His recent form is an indication he is continuing to improve. After all the stick he has received, Carroll deserves his chance to shine.
A price tag of £35 million is difficult to escape, especially if you play poorly.
Despite his recent improvement, most of Carroll's time at Liverpool has been spent under performing. In 19 appearances for Newcastle during the 2010-11 Premier League season, he scored 11 goals.
For Liverpool, he scored 11 in 58 appearances across all competitions.
Signing for such a club is undoubtedly the pinnacle of most careers. Liverpool's reputation has been built on years of success, numerous trophy wins, and the production of world-class talents such as Steven Gerrard and Robbie Fowler.
The club's fans expect greatness.
Anfield is a relentlessly hostile stadium for any opposition. The problem is, this hostility can often get to the home side. Carroll wasn't the only player who succumbed to the pressure during his spell in the first team.
He was, however, the only individual weighed down by the burden of that outrageous transfer fee.
Upton Park will welcome Carroll's arrival. He may now receive the opportunity to play without having his every action scrutinised. If he can showcase his talents at the same time, Liverpool fans will welcome him back with renewed confidence.
It's no secret Brendan Rodgers likes to play a passing game.
His spell at Swansea City was defined by the implementation of a slick style that focused on the retention of possession.
Summer signings such as Fabio Borini, Nuri Sahin and particularly Joe Allen indicate Rodgers is shaping Liverpool to take on a similar mantra. After just three appearances for the Reds, club captain Steven Gerrard has praised the latter's impact (via The Daily Star).
It is unlikely Rodgers will ever base Liverpool's attack around a centre forward like Carroll.
The acquisition of Borini indicated as much. The Italian's partnership with Luis Suarez is already beginning to blossom. Both play to the feet of their teammates, providing smart link-ups and neat passing to unlock a defence. With Allen sitting behind, the midfield is being trained to do the same.
Carroll would have offered Liverpool an alternative this season.
His aerial ability might be useful against physical teams. Sometimes a different approach is needed, whether it be a long ball into the box or someone to offer extra intimidation from both defensive and attacking set pieces. Carroll is the man to do that.
Unfortunately for Liverpool, if Borini and Suarez hit a patch of bad form or get injured, Plan B doesn't exist.
Having worked with Andy Carroll during his time at Newcastle, Sam Allardyce knows what to expect from the striker.
Earlier on in the transfer window Carroll made it clear he wanted to stay at Liverpool (via ESPN). I believe his relationship with Allardyce finally sealed the moved away.
Big Sam is fond of utilising centre forwards the 'old fashion' way.
While it's unfair to suggest his Bolton and West Ham sides have been built around the long ball, it's a significant part of their game. Before Carroll's arrival, Carlton Cole represented the Hammers' most intimidating force.
In the 2010-11 Premier League season, where West Ham got relegated, Cole scored five goals from 35 appearances. So far this year, he hasn't hit the back of the net.
It's apparent the team need an injection of life in the striking department, something Carroll can provide.
If Allardyce shows the same faith in Carroll as he did at Newcastle, the loan signing will be more than justified with goals.
Every newly promoted side is looking for a hero.
West Ham is full of experience. Players such as Kevin Nolan and Jussi Jaaskelainen will be invaluable as the Premier League season progresses.
In the opening two fixtures, none of the club's registered strikers have scored. The recent 3-0 loss away to Swansea raised many questions about the offensive line.
Matt Jarvis was the only player to perform adequately, but his clever work on the wing often went to waste. When Jarvis crossed, nobody was there to finish.
Now Carroll is at the club, this should change.
He'll work hard to get into the box and will be strategically placed to impose his physicality. The tall forward can dominate the best if he's at the spearhead of an attack.
If Carroll manages to score consistently, his loan move will be one of the summer's biggest coups. If he fires West Ham to a second successive season in the Premier League, he'll be hailed as a hero.