The Spaniard replaces Arsene Wenger in the Emirates hotseat after the Frenchman stepped down after 22 years in north London.
Emery, 46, left French side Paris St Germain at the end of their campaign following a two-year stint during which he won one Ligue 1 title and four domestic cups.
Previously, Emery led Sevilla to three successive Europa League trophies - a proven track record which seemingly saw him emerge as the preferred candidate ahead of former Arsenal captain Mikel Arteta for the Emirates Stadium job.
The new manager said: “I am thrilled to be joining one of the great clubs in the game. Arsenal is known and loved throughout the world for its style of play, its commitment to young players, the fantastic stadium, the way the club is run.
"I’m very excited to be given the responsibility to start this important new chapter in Arsenal’s history. I have met Stan and Josh Kroenke and it’s clear they have great ambitions for the club and are committed to bringing future success.
"I’m excited about what we can do together and I look forward to giving everyone who loves Arsenal some special moments and memories.”
On Tuesday evening, the homepage of Emery's personal website displayed a picture of the Spaniard alongside an Arsenal crest.
It carried the message: "Proud to be part of the Arsenal family."
The image was taken down from Emery's website shortly after 11pm, replaced by an error message.
However, Emery was officially unveiled as their new manager on Wednesday on a lucrative deal worth £6million-a-season after agreeing to replace Wenger at the helm.
It is understood Arsenal conducted in-depth interviews with several candidates from a long list of potential managers.
Luis Enrique, the former Barcelona coach, ex-club captain Patrick Vieira and Juventus' Massimiliano Allegri were among those strongly linked.
However, it is believed no one pulled out of the race and the Gunners have settled on their preferred choice.
Arteta, currently assistant manager under Pep Guardiola at Manchester City, is said to have expressed reservations over how much control the new head coach would have in transfers.
Emery, though, has worked in such models previously, enjoying great success at Sevilla with sporting director Monchi, who is now at Roma.
Majority owner Stan Kroenke said: “We’re delighted to welcome Unai to Arsenal. He’s a proven winner. We’re confident that he is the right person for the job and that he will work to deliver the triumphs our fans, staff and everyone who cares about Arsenal want.
“Several things stood out during his interview and the entire process; his football knowledge, energy, determination and love of the game.
"His familiarity with our club and our players, the Premier League and the game in Europe were all very impressive. He shares our vision to move forward, to build on the platform created by Arsène Wenger and help this club enjoy greater success.”
Assistant Juan Carlos Carcedo would likely be joining the Spaniard as part of a revamped backroom staff, which could yet see Steve Bould and coach Jens Lehmann stay on at the club.