Allardyce watched from the stands as Wayne Rooney scored a hat-trick in Everton's 4-0 thrashing of West Ham United on Wednesday and will be in place for the weekend's home clash against Huddersfield Town.
"Sam has my full backing, the board have made their decision and, like any manager, he needs to be afforded time, patience and given the chance and the backing from the whole football club and he certainly has my full backing," caretaker boss David Unsworth said after signing off with a smile on his face.
Unsworth, who will return to his job with the under 23s having taken charge of the first team since Ronald Koeman's sacking in October, said he had spoken to Allardyce at the club's training ground on Wednesday.
"I had 10 minutes with him this afternoon at Finch Farm, he came in with Steve Walsh and had a quick look around," he said.
The 63-year-old Allardyce, who has had stints at Sunderland, West Ham United and Crystal Palace after an impressive eight-year stay at Bolton Wanderers, takes over with Everton 13th in the table, five points above the relegation zone.
Allardyce has never been relegated from the Premier League and stepped in to rescue struggling Crystal Palace midway through last season. He resigned, however, before the start of the current campaign.
Everton sacked Koeman last month and placed Unsworth in temporary charge, but results did not immediately improve.
Allardyce, who has a reputation for building direct, defensively solid teams, has been out of work since leaving Palace in the close season.
That job was his first role since resigning as England manager in 2016 following a newspaper sting after just 67 days and one game in charge.
Everton have struggled this campaign despite the club spending more than 140 million pounds on new players during the close-season.
That spending splurge included a club-record 44 million pounds on Gylfi Sigurdsson, 25 million on goalkeeper Jordan Pickford, 25 million on defender Michael Keane and 24 million on Dutch international midfielder Davy Klaassen.
While they began the season by beating Stoke City in their opening Premier League game and earning an impressive point away to Manchester City, their form then unravelled with seven defeats in their next 12 matches in all competitions.
They suffered heavy losses to Tottenham Hotspur, Atalanta, Manchester United and a 5-2 home thrashing at the hands of Arsenal, which led to Koeman's sacking.
Goals have proved harder to come by for Everton than last season following the departure of striker Romelu Lukaku to Manchester United in July.
Allardyce's task will be to restore stability and return the club to being regular top-seven finishers as they were under David Moyes from 2004-2013 and under Koeman last season.
(Reporting by Matt Westby in London and Shrivathsa Sridhar in Bengaluru; Editing by Toby Davis)
