Perez, 29, joined Arsenal from Deportivo La Coruna in August 2016 and spent last season on loan back at the Spanish club.
“I’m very excited, as West Ham are giving me the chance to stay in the Premier League and I’m very happy to be part of this new project,” Perez said on West Ham’s official website.
“West Ham have great fans and play in an amazing stadium, so to sign here brings me great joy. I’m very motivated.”
Perez scored seven times in 21 appearances for Arsenal, including a hat-trick in the Champions League away to Basel.
However, the striker then opted to return to Deportivo on loan last season in search of more regular first-team football.
In a message on his personal Twitter account, Perez said: “I would like to thank Arsenal fans for their support, they always showed me great affection and love from the first time I arrived in London.
“I also want to give my thanks to my team-mates and to the staff of this club for giving me the opportunity to make my debut in one of the best leagues in the world and win the FA Cup.
“I wish you all the best for the future and I hope you can go on and achieve great things.”
The fee for the transfer is believed to be worth around £4million, plus add-ons.
Perez’s switch to the London Stadium sees him reunited with former Gunners midfielder Jack Wilshere, who joined West Ham following the end of his contract.
New Irons boss Manuel Pellegrini has now brought in nine players during the summer window, spending more than £100million, as he looks to reshape a squad which battled for survival during most of the previous campaign under David Moyes.
West Ham director of football Mario Husillos is confident Perez will prove another valuable addition.
“Lucas is a very experienced player that Manuel and I both know well from Spain,” Husillos said on the club website.
“He has great technical quality, and can play as both the main striker or the second striker in a deeper role. He scores goals, but can also create them and link well with the midfielders.
“His style is different from the other strikers we have available to us, and that gives the manager more options for our attacking play.
“Lucas also has experience of the Premier League with Arsenal, and is familiar with life here in London, which of course means that he will take less time to adapt and be ready.”
The 23-year-old on Wednesday joined from Athletic Bilbao for £72.1million, after Chelsea met his €80m buyout clause. Kepa could make his Chelsea debut in Saturday’s Premier League clash at Huddersfield.
The recruitment of the Spain international on a seven-year contract allowed Thibaut Courtois to depart for Real Madrid for €35m (£31.5m), with Croatia midfielder Mateo Kovacic moving in the opposite direction on a season-long loan.
Kepa told Chelsea’s official app, The 5th Stand, on Thursday: “All clubs need to fight to win titles and what all players want is to win. So what I would like is to win more titles for the club.
“This is what I aspire to do. I would like to be remembered at this club when I leave years down the line as a goalkeeper that has helped the club win a lot of titles and gone down in history in that way.”
Courtois won two Premier League titles in four seasons as Chelsea’s first-choice goalkeeper but had a year remaining on his contract and had expressed a desire to return to Spain.
He had spent three seasons on loan at Atletico Madrid from Chelsea before displacing Petr Cech as the Blues’ number one.
I am really happy and excited to be here @ChelseaFC. It is a privilege and an amazing feeling to be part of this club and I can't wait to start with trainings and matches. I will try to do my best for this club and hope to bring joy to our fans. pic.twitter.com/OMydrc2i3j— Mateo Kovačić (@Mateo_Kova23) 9 August 2018
Now Kepa is Courtois’ successor, after speaking to compatriots Cesar Azpilicueta, Alvaro Morata and Pedro about life at Stamford Bridge.
He said: “I had excellent references from them about the club, knowing them and being able to talk to them has been a great support for me over the last few days.
“And then of course there are many great players here, like N’Golo Kante, (Eden) Hazard, (Olivier) Giroud. It’s a squad full of fantastic players.
“We have a very exciting season ahead of us, so let’s go for it.”
Chelsea finished fifth in the Premier League last season in defence of the title they won in 2016/17, missing out on Champions League qualification.
They won the FA Cup in Antonio Conte’s final match as head coach before the Italian was belatedly sacked in July, with Maurizio Sarri appointed in his place.
Hazard’s future has been the subject of debate, with the Belgium playmaker stalling on an offer to extend his contract, which presently expires in June 2020.
The Blues appear to have held on to Hazard, for now. Kovacic’s capture, meanwhile, was completed on Thursday morning.
The 24-year-old former Dinamo Zagreb and Inter Milan midfielder was described as “a perfect fit” by Chelsea director Marina Granovskaia.
She told Chelsea’s official website: “His style is well suited to how Maurizio wants to play and we are sure he will prove to be a valuable asset for the club.”
Kovacic said: “I am really happy and excited to be here at Chelsea. It is an amazing feeling. I will try to do my best for this club.
“It is a new league for me, the beginning is always tough but I am sure the coach and my new team-mates will help me and I am looking forward to a great season.”
There could be moves, on loan and permanently, for a number of Chelsea squad players on Thursday’s final day of the summer transfer window, although any moves into continental Europe can be completed before the end of the month.
Tottenham boss Mauricio Pochettino says it will be “difficult” to make any signings before the transfer window shuts.
Spurs are the only Premier League club not to make an addition to their squad and Pochettino admits any arrivals are unlikely before the 5pm deadline (8pm UAE time).
He said: “It will be difficult. It’s not over until the game is over but with four hours to go it will be difficult.”
The Argentinian refused to speak about Spurs’ pursuit of Jack Grealish, but after a £25million bid was rejected on Wednesday that deal appears dead.
With just hours to go until the window shuts, it looks like being a fruitless period for the club, who Pochettino challenged to be “brave and take risks” at the end of last season.
The Argentinian said: “I think it’s so difficult now because it’s only a few hours until the end and I don’t have news now.
“It’s sure it will be so difficult. Always, until you finish the game, you cannot say, ‘Now it’s the end of the game’. But in four hours it will be difficult to sign some players.”
The tonic to that lack of transfer activity is that Spurs have been able to keep all of their top stars, with a number of them tied down to new deals.
The window remains open to European clubs until the end of August, so there is still the possibility of players going out even after the English deadline passes.
Toby Alderweireld, Danny Rose and Mousa Dembele could still leave in the next few weeks, but Pochettino has been able to keep hold of the likes of Harry Kane, Christian Eriksen and Kieran Trippier.
Alderweireld was heavily linked with a move to Manchester United but any firm interest never materialised.
“We have a very good squad, with very good players and it is very difficult to add players to that,” the boss added.
“We didn’t sell players and with 25 players in the squad it is difficult to add players.
“It’s not about adding because it’s fashionable to sign players. It’s about if you really need them or not and then if you can get your target or not.
“It’s true our targets are always the same level as Manchester United, Manchester City or Real Madrid but for different circumstances we can’t achieve that.
“I am not worried, I am not sad, I am so happy. To keep our best players was our objective and goal and we have achieved that – at the moment, we will see what happens in the next 20 days in Europe.
“If you can’t add a player that can improve your squad, the most important thing is to make sure you do not lose your best players and I think that was a great job from the club to keep the best players and the manager.”
Pochettino does accept how the lack of activity might be viewed externally, but is happy with the club’s vision.
“Of course it’s difficult to understand for people in football that Tottenham didn’t sign or sell players, but sometimes in football you need to behave differently,” he said.
“If we are happy with our squad and cannot improve our squad, sometimes it’s better to keep our squad together.
“There’s no reason to not trust in our squad and no reason not to believe in our club or believe in what we can do.
“The most important thing is we’re so positive and of course we’re going to challenge for big things this season too.”