Atherton is the third high-profile former Conservative to join the part in Wales, after former Brexit minister David Jones and Conservative Member of the Senedd Laura Anne Jones.
Reform is currently deciding who will be its candidates next May.
Under Wales’ new election map, 16 seats will each elect six members, chosen from party lists.
Atherton left the Conservative Party in August having said the previous year that the Conservatives should “embrace” Nigel Farage and Reform.
At the time she said the Conservatives were “impotent” and “no longer align with her values or ideology”.
Having served as Wrexham MP from 2019 until she lost her seat in 2024, Atherton was parliamentary under-secretary of state for defence people, veterans and service families for a brief period in 2022.
She was the first Conservative politician to represent the Wrexham constituency since its creation in 1918.
