The former Blue Peter presenter will team up with Alex Jones from February 14, every Monday to Thursday. Chris Evans will continue as Jones's co-host on Fridays.
"I'm over the moon to be joining The One Show as a full-time presenter. The BBC has made everything possible so I can also continue presenting Countryfile," said Baker. "It's the ideal combination for me and I can't wait to get started."
Alex Jones commented: "I'm so pleased to be welcoming Matt to The One Show as a permanent fixture. We have hosted the show together many times before, he's fantastic to work with and the perfect choice for The One Show."
BBC One controller Danny Cohen added: "With such a wide range of experience including Blue Peter, Countryfile and Strictly Come Dancing, Matt's perfectly placed to take his place on the One Show sofa alongside Alex Jones."
The One Show has featured a selection of guest hosts since Jason Manford stepped down from the programme in November.