The number has special significance to the Netherlands midfielder as it was the one he wore when he first started at the Dutch club.
Van de Beek completed a deadline day move to the Toffees on loan to escape from his frustrating time at Manchester United.
"I saw number 30 was free and I started with this number at Ajax," Van de Beek said.
On Twitter, Ajax said: "Back in the days. Go get em, Don!"
He will link up at Everton with his former Ajax and Netherlands youth teammate Anwar El Ghazi, who the club signed from Aston Villa.
El Ghazi will wear number 34 at Goodison Park in tribute their former teammate Abdelhak Nouri.
In July 2017, while El Ghazi and Van de Beek were at Ajax, Nouri collapsed and suffered a cardiac arrhythmia attack during a pre-season match for the Dutch side.
The incident left the midfielder, who was 20 at the time, with permanent brain damage and went into an induced coma.
Nouri woke from his coma in 2020 after two years and nine months.
Van de Beek actually wore the number 34 shirt at Man Untied in tribute to his old teammate.
He actually called Nouri's father, Mohammed, to ensure such a tribute was acceptable - with Appie hearing Van de Beek and reacting to the shirt number choice.
Mohammed said: "Donny called me. He asked if I was okay with him playing with No.34.
"And I said he should. I can only be proud of that, it's a nice gesture, very special for us.
"Appie's eyes looked very sharp suddenly.
"He had tears of joy on his cheeks and was clearly moved when he heard Donny's voice. Donny is like a son to us.
"Everyone at our house was very emotional, happy with Donny's wonderful transfer.
"It was a special day."