Kay will succeed the Bruce as host of the BBC mid-morning show when he leaves this spring.
The presenter will launch the new show in May. Speaking about his appointment, Kay said: “I’m absolutely over the moon to be handed the microphone to present the mid-morning show on Radio 2, and what an honour to follow in the footsteps of the mighty Ken Bruce.
“I look forward to playing some of the best music in the world whilst in the company of the Radio 2 listeners who I feel I’ve got to know over the last 18 months. It’s a dream come true to join the Radio 2 family and I can’t wait to start.”
Bruce will leave the corporation after 45 years to host a show on commercial rival, Greatest Hits Radio.
The 71-year-old has been on the station since 1984, but told listeners in January: “I’ve done everything it is possible to do at Radio 2.”
He has hosted the 9:30am to 12pm weekday show since 1990, and been with the BBC since 1977 where he began as a presenter on Radio Scotland.
His decision comes after Steve Wright was nudged out his Radio 2 afternoon slot, with Simon Mayo, Graham Norton and Paul O’Grady also moving on in recent months.
“After 45 years of broadcasting on BBC Radio, it’s time for a change,” Bruce said in a statement.
“I would stress that this is entirely my decision. I will always be very proud of my association with the BBC and Radio 2.”
Bruce, who said “the time was right” to quit, has now joined the team at Greatest Hits, where he will begin working in April.
He will present a show from 10am to 1pm on the Bauer radio station, alongside presenters including Simon Mayo at Drivetime and Jackie Brambles in the early evening.
“Nothing stays the same forever and I have decided the time is right for me to move on from Radio 2 when I reach the end of my current contract in March,” Bruce said.
“It’s been a tremendously happy time for me. I’ve made many friends and worked with many wonderful colleagues.”
He continued: “Some new opportunities have come up and I would like to continue my career in a slightly different way in the next few years, the details of which will be revealed shortly.”
Bruce also thanked “everyone who has helped to make the mid-morning show a success”.
He described his new job with Greatest Hits as “a new adventure” and assured his long-time listeners about the fate of his beloved quiz PopMaster.
“I’m looking forward to getting started and to you joining me for my very first show,” he said.
More to follow