Rice (centre) has become Arsenal's record transfer following his move from West Ham
The Gunners' announcement comes after the 24-year-old issued an open letter to Hammers fans saying how 'tough' a decision it had been to leave the club.
Rice has signed a five-year contract with Arsenal in a deal which has the option of a further year.
"I've been looking at Arsenal over the last couple of seasons and the trajectory they've been on," he said.
"Not last season but the season before, they [only] finished fifth, but you could see the style of play Mikel was implementing.
"Last season was an outstanding season, blowing pretty much every team out of the water with the exception of Manchester City."
Rice has moved on after captaining West Ham to the Europa Conference League title last season.
He is Arsenal's third major signing this summer as they look to go one better than their second-placed finish in the Premier League last season.
His arrival follows the recruitment of Germany international Kai Havertz from Chelsea in a £65m deal and Netherlands defender Jurrien Timber from Ajax for a fee which could rise to £38.5m.
"He is a player with tremendous ability, who has been performing at a high level in the Premier League and for England for a number of seasons now," said Arsenal manager Mikel Arteta.
"Declan is bringing undoubted quality to the club and he is an exceptional talent who has the potential to be very successful here.
"Declan has great experience in the Premier League at only 24 years old. He has captained a very good West Ham team and as we all saw, he recently lifted a European trophy.
"The responsibility and role he has taken on has been very impressive and we are really excited that he is joining us."
Arsenal succeeded with their third bid for Rice, having had two earlier offers rejected which were below West Ham's £100m asking price.
Premier League champions Manchester City withdrew from trying to buy the player after they had a £90m offer turned down.
The initial fee for Rice equals the £100m City paid Aston Villa for Jack Grealish in 2021.
The Premier League's record transfer is the £105.6m Chelsea paid Benfica for Enzo Fernandez in January 2023.
Earlier this summer, Jude Bellingham moved from Borussia Dortmund to Real Madrid for an initial £88.5m in a deal which could rise to £115m with add-ons.
Rice scored 15 goals in 245 games for West Ham, having joined the club as a 14-year-old following his release 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 as a substitute against Burnley, going on to make his full debut that August in a start against Southampton.
Rice became the club's captain following Mark Noble's retirement in 2022 and, in what proved to be his final season at the London Stadium, was named the Europa Conference League's player of the season as West Ham ended their 43-year wait for a major trophy.
He has been capped 41 times by England, playing in all five of their matches at last year's World Cup.
"In football, amazing opportunities arise. Big clubs, like Arsenal, have come for me and it's really hard to turn down," said Rice.
"You only ever get one career and I really believe in what Mikel is building here and the squad he's building. I'm really looking forward to the future with Arsenal.
"For me as a player, I've come here really hungry to have more success and to spend my best years at this great club."
He added: "[Arteta] is a massive factor in why I've come here.
"I know he's going to get the best out of me. I know I've got more levels to go up in my game, and I feel like he's the manager to take me to those next levels.
"I'm really excited to be working with him."
-- Courtesy of BBC Sport