Derry-Evans, who joined the building society’s board in June, has experience of leading an international legal practice in Cameron McKenna, while he is also the chair of Bath-based law firm Omnia Legal.
Moorsom had originally planned to retire in April, however following Terry Fussell’s illness, who was due to become chairman, he continued until the end of the year.
Dick Jenkins, chief executive at Bath Building Society said: “Robert has some excellent qualities and experience that will be valuable to the society in his new role.
“I look forward to working with him in this new capacity.”
Derry-Evans sits on the Nomination and Remuneration Committee at Bath and the Board of Trustees at Cheltenham-based medical charity Cobalt Health.