The 19-year-old played 33 matches for the Saints last season, and although the fee was not officially revealed, it has been reported that Everton would pay around £42 million for the player.
"I think I’ve come to Everton at the perfect time and hopefully I can be here for many years," Dibling said.
"My aim for this season is just to play as many games as I can and to help the team as much as I can with goals and assists."
Everton have started the new season with one win and one loss in the Premier League, and will next play Mansfield Town in the League Cup on Wednesday before visiting Wolves in the league on Saturday.