Sam Allardyce says he is "enthused and energised" about returning to management with Everton.
The 63-year-old former England boss has signed a deal until June 2019 to become the permanent successor to Ronald Koeman, who was sacked in October with the club 18th in the Premier League.
Allardyce has been out of work since leaving Crystal Palace in May.
"The ambition of the club, allied to all the fantastic history, has proved to be key for me," he said.
"Obviously, the club has gone through a difficult spell and hopefully I can put that behind us as quickly as possible and start looking upwards again.
"I think that whatever's happened before we've just got to all rally around together. Our will to do it all together would be my way forward.
"I feel really enthused and energised to come in as manager."
Dutchman Koeman guided Everton to seventh place last season - his first in charge - but was sacked the day after a 5-2 home defeat by Arsenal on 22 October.
The Toffees spent more than £130m on new players in the summer but have struggled without last season's top scorer Romelu Lukaku, who was sold to Manchester United for £75m in July.
Under-23 coach David Unsworth, who was named as talent development coach of the year at the UK Coaching Awards on Thursday, was placed in temporary charge, and won two of his eight matches.
His final game before returning to his former role was Wednesday's 4-0 Premier League victory over West Ham at Goodison Park, which Allardyce watched from the stands.
"Sam understands the long-term ambitions we have for this great club and I know he is a man who gives it his all and is focused 24 hours a day," added Everton's majority shareholder Farhad Moshiri.
"I would like to put on record our thanks as a club to David Unsworth for stepping up when we needed him over the last few weeks. He is a real Evertonian, embodying the commitment, pride and passion we all feel for the club."
Everton are now 13th in the league and face Huddersfield on Saturday.
Allardyce left Palace after just five months - his first job since an ill-fated one-game spell as England manager - having led the club to eight wins in 21 games to secure a 14th-place finish in the league.
BBC Sport's chief football writer Phil McNulty
Allardyce is not Everton's first choice - how can he be? If he was, he could have been appointed a month ago.
Results and performances, with Wednesday's 4-0 win over West Ham aside, have been so lamentable that alarm bells started to ring loud and clear inside the Goodison Park boardroom - rekindling almost forgotten interest in 'Fireman Sam'.
No matter how Everton dress this up, it is an appointment made out of desperation and serious necessity, not with the long-term vision owner Farhad Moshiri has always had in mind.
Allardyce facts and figures
- Everton will be Allardyce's seventh Premier League club - more than any other manager. Harry Redknapp is next on the list with five.
- He has never been relegated from the top flight as a manager and was promoted to the Premier League with Bolton and West Ham.
- He has a Premier League win percentage of 33.8%.
|Sam Allardyce's Premier League record|
'Not exciting, but what Everton need'
Former Everton midfielder Leon Osman told BBC Radio 5 live: "Does the Allardyce appointment excite me? No. Is it what we need right now? Probably, yes.
"If games are won, then fans will warm to him. Long term, maybe not.
"Like at Bolton, he has some quality players here so hopefully he can produce something similar at Goodison."
Ex-Everton defender Alan Stubbs said: "The biggest thing is the cloud of uncertainty around the club has been removed.
"There's certainly enough quality there for Everton to still finish in the top 10. I'm sure Everton will bring in a striker in January. It's been the one thing people have been bemused by - why the club didn't address it and it's the one area they've been lacking.
"Sam is a very astute manager, he'll quickly rectify areas of the team that need addressing. Apart from the England job, I think this is probably the most exciting, challenging job that Sam has encountered and I'm sure he'll be really looking forward to it."
What he said in May...
When Allardyce left Palace last season, where he enjoyed his highest win percentage in the Premier League, he said he had "no ambitions to take another job".
He added: "I want to be able to savour life while I am still relatively young, and when I am still relatively healthy enough to do all the things I want to do - like travel, spend more time with my family and grandchildren without the huge pressure that comes with being a football manager.
"I simply want to be able to enjoy all the things you cannot really enjoy with the 24/7 demands of managing any football club, let alone one in the Premier League."