Baker had hosted alongside Jones for nine years until his exit, which was made so he could spend more time with his family.
Jenas, a former professional footballer who once won the League Cup with Tottenham Hotspur, will host the series with Jones from Monday to Wednesday.
In a statement, Jenas said: “I’ve absolutely loved being a part of The One Show, Alex, the team and the viewers have made me feel so welcome and we’ve had a lot of fun already - I’m really looking forward to joining as a full-time host alongside Ronan.”
Keating, meanwhile, will present with Jones on Thursdays and Fridays. Keating is a Radio DJ, singer and songwriter known for serving as a judge on The X Factor and a coach on The Voice.
“I’m so delighted to officially be a part of The One Show family,” he said. “It’s a show I’ve loved for many years as both a guest and a host and I can’t wait to get started alongside Alex, Jermaine and the wonderful team behind the scenes.”
Both presenters are already familiar to One Show audiences, having appeared in a non-permanent capacity on the programme many times before.
They will officially start on the week of Monday 10 May.
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