The Toffees triggered the Senegal midfielder's release clause - which is believed to be £7.1million - and he has joined the Blues on a four-year deal to June 2020.
Marseille also triggered the release clause but Gueye chose to remain in the Premier League with Everton and became the club's second signing of the summer after goalkeeper Maarten Stekelenburg joined from Fulham.
Who is Idrissa Gueye?
Although he was unable to prevent Aston Villa being relegated to the Championship, Gueye was one of the few success stories at Villa Park last season. The 23-year-old was second only to Leicester City's N'Golo Kante in terms of the number of combined interceptions and tackles made across Europe's top five leagues.
He played 37 games in his one season with the Midlands club after joining from Ligue 1 side Lille last summer for £9million. He made 134 appearances for Lille making his breakthrough in 2010 and contributing to their Ligue 1 title success in 2011. He established himself as a regular starter over the following four seasons, competing in two Champions League campaigns during that time.
The 26-year-old is a combative, dynamic, energetic and technically gifted midfielder, who has been capped on 32 occasions by Senegal. His primary job as a defensive midfielder is to protect the backline by breaking up play, tracking runners and providing a reassuring presence in his role as the enforcer. However, he has been happy to take on more attacking duties and play slightly more advanced.
Steve Walsh's influence already being felt at Goodison
Everton's new director of football, Steve Walsh arrived at Goodison Park with an expert knowledge and understanding of the market. Walsh was instrumental in Leicester parting with £5.5million to sign N'Golo Kante last summer and it proved to be one of the best buys of the summer.
Gueye's transfer to Everton has the potential to be a bargain - his statistics from last season speak for themselves - and is an example of how Walsh's influence is already being felt at Everton. Gueye is a player that Walsh was tracking at Leicester before signing Kante and when Koeman and he met following Walsh's appointment to identify transfer targets, Gueye was believed to be one of the first names on Walsh's list.
With Walsh's arrival and greater financial clout available following Farhad Moshiri's investment earlier this year, this could signal the start of Everton's major transfer business, with Swansea City captain Ashley Williams expected to follow Gueye to Merseyside this week.
Player and manager delighted with new signing
Gueye told evertontv that he was "very happy and excited" to move to Everton after a "hard season" at Aston Villa. When asked what type of player he is, the Senegal international said a "defensive midfielder" who "likes making passes and getting forward". He added that he believes his combative playing style suits the Premier League.
His new boss was also delighted with his latest signing and said that it conforms to his goal for the transfer window of "increasing the quality" in the team. Koeman referenced Gueye's impressive statistics and record from last season and said that his attributes are the qualities that Everton "need".