Advice network Lighthouse Group will now provide advice to members of the Public and Commercial Services Union.
The contract for financial advice, retirement planning, mortgage and investment advice will be for three years, starting from 1 June.
The union has 200,000 members across the UK and is the largest civil services trade union.
Lighthouse chief executive Malcolm Streatfield says: “We very much look forward to building on our relationship with PCS over the years ahead. The board is very pleased to have secured this contract.”
Lighthouse recently renewed its contract with the UK Royal College of Nursing for three years, as well as signing to provide advice to employees at guidance body the Money Advice Service.