Jones, who has over ten years' experience fronting various television and radio programmes in Wales, will join Jason Manford and Chris Evans when the revamped version of the show is launched on August 16.
"I'm absolutely thrilled to bits to be joining The One Show," said Jones. "I watch the show at home every night on my sofa and can't believe I'm now swapping it for the One Show sofa!
"It is the break I have only ever dreamed of and can't believe I'm getting to work with Jason Manford, who I think is just fantastic - it truly is the icing on the cake."
Jones has already received praise from the corporation ahead of her debut, with Controller of Daytime Liam Keelan describing her screen test with Manford as "outstanding", while One Show editor Sandy Smith predicted that viewers will easily warm to the new arrival.
"Alex is warm, funny and very experienced despite being almost unknown outside Wales," Smith said. "It feels right for many reasons to be developing someone new at this time and I hope One Show viewers enjoy seeing her relationship with Jason develop on screen and take her to their heart."
The One Show is currently being fronted by a series of guest presenters following the well-publicised defection of original hosts Adrian Chiles and Christine Bleakley to ITV.
The pair will reunite to front Daybreak, a revamped version of the channel's long-standing breakfast show GMTV.