Everton have completed the signing of midfielder Idrissa Gueye from Aston Villa on a four-year deal.
The Toffees triggered the 26-year-old's release clause - which is believed to be £7.1m - and the player travelled to Finch Farm on Tuesday to finalise the move.
Marseille also triggered the release clause but Gueye chose to remain in the Premier League with Everton.
Gueye played 37 times for Villa last season following his £9m switch from Lille, but was unable to prevent them being relegated to the Championship.
The Senegalese international becomes Koeman's second signing since he took over from the sacked Roberto Martinez in mid-June, having brought in goalkeeper Maarten Stekelenburg from Fulham.
"Idrissa will provide quality in our squad in the midfield and we want to increase the quality in the team," said Koeman.
"He was one of the players last season with the best record in interceptions and pressing in the midfield. We need this kind of quality and I'm happy to have him joining our club."
Gueye was second only to Leicester's Premier League-winning midfielder N'Golo Kante in terms of the number of combined interceptions and tackles made across Europe's top five leagues.
"I am very happy to be here at Everton and I am very happy and excited to put on the Blue shirt to start the new season here," he said.
"Everton is a big team with big players and great fans. The manager told me he is very happy to have me here and that we will have time to talk about football after.
"I am a defensive midfield player and I like making passes and also getting forward.
"I have played a season with Aston Villa which was a hard season but I think my style is good for the Premier League.
"You have to fight every game. All the teams are strong and you have to concentrate all through the game.
"Goodison Park is a very good stadium and the fans are very good too. They push their team forward and I am excited to play there."