Dibling, who has been capped twice by England Under-21s, made his Southampton debut as a 17-year-old in August 2023.
He went on to make 44 appearances in total for the Saints, scoring four goals, playing predominantly as a left-footed winger on the right flank or as an attacking midfielder.
Dibling, who has taken the number 20 shirt, said the chance to play under Moyes was a major part in his decision join the Merseyside club.
“I think the manager is perfect for me,” he added.
“Obviously he’s helped so many players before who were in my position. I’ve spoken to him and I know he’s going to be good for me.
“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. As a team, hopefully we can have a good season and see where it goes.”
Moyes said Dibling is a “talented young English player” with “lots of potential”.
“Tyler adds to our attacking options and we will support him in his future development,” he added.
“We are eager to build a team that Evertonians can enjoy watching and be proud of, and Tyler can help us achieve that.”
Former Southampton and Everton winger Theo Walcott said it was a “good deal” for Everton.
“I think the fact they are in the new stadium, he will thrive there. I’ll be intrigued to see where he fits in because he will want to start,” Walcott told the Monday Night Club.
“When you play for Everton you have to run. At times, in a very poor Southampton team he was working his socks off, but it’s a different kind of pressure, playing for Everton Football Club.
“I do hope it goes well for him. The new stadium, new players and personnel, it fits the bill for me, it’s a really good move.”