England midfielder Declan Rice says it has been a "tough" decision to leave West Ham but that his departure "is about my ambition to play at the very highest level of the game".
Rice issued a letter to Hammers before his transfer to Arsenal for £100m plus £5m in add-ons was confirmed.
West Ham also confirmed the 24-year-old was leaving the club.
"The last few days and weeks have been an absolute whirlwind of emotion," said Rice.
"You have taken me into your hearts as one of your own from day one. Even when it was just a handful of fans at the Under-18 or Under-23 games, I felt that love, and it has just grown stronger as the years have passed.
"Playing in front of you has been an honour, we have had such great times together, and you all mean so much to me.
"I want you to know how tough a decision it has been for me to leave an environment that I have loved and cherished so much.
"Ultimately, though, it has only ever been about my ambition to play at the very highest level of the game."
Rice scored 15 goals in 245 games for West Ham, who he joined as a 14-year-old after being released from Chelsea's academy.
He signed his first professional contract with the Hammers in December 2015 and made his first senior appearance on the last day of the 2016-17 Premier League season against Burnley, going on to make his full debut that August against Southampton.
He was named the club's captain following Mark Noble's retirement in 2022 and, in what proved to be his final campaign at London Stadium, was named the Europa Conference League's player of the season as West Ham ended a 43-year wait for a major trophy.
"Playing on the opposite team to West Ham for the first time will be an unusual experience," added Rice.
"I'm not sure yet exactly how I will feel, but I also know you will all understand and respect that my professional loyalties have to now lie with my new club.
"I will always give 100% every single time I pull on the shirt - because that is how I have been brought up at West Ham, and I'm sure you wouldn't expect anything else."
Arsenal succeeded with their third bid for Rice, having had two offers rejected after falling short of the £100m West Ham wanted.
Premier League champions Manchester City withdrew from trying to bring in the player after they had a £90m offer turned down.
The Gunners are set to add Rice, having signed Germany midfielder Kai Havertz from Chelsea in a £65m deal and Netherlands defender Jurrien Timber from Ajax for a fee that could rise to £38.5m.
"I would like to make it clear to our supporters that we did not want to sell Declan," said Hammers joint-chairman David Sullivan.
"We wanted to build our team around him and made a series of improved, long-term contract offers to secure his future.
"However, once Declan made it clear that he wished to move on and seek a new challenge, the club felt it would not be right to stand in his way, acting in the best interests of West Ham United.
"He leaves with the sincere thanks, respect and love of everyone here, having earned his place in history alongside the great Bobby Moore and Billy Bonds as the only players to have captained West Ham United to a major trophy.
"More than that, he has always given absolutely everything for the football club throughout his time with us.
"Declan will forever hold a special place in the hearts of Hammers fans and everyone here at the club."
West Ham, in attempting to replace Rice, could try to sign Mexico midfielder Edson Alvarez from Ajax.
The Hammers are interested in the 25-year-old, who is set to play for his country in Sunday's Gold Cup final against Panama.
Alvarez joined Ajax in 2019 and had been linked with a move to Borussia Dortmund, who backed off because of the Dutch club's demands.
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