The voluntary charter, part of the Trades Union Congress' "Dying to Work" campaign, sets out a process to support staff with a terminal diagnosis.
It also aims to ensure their employment and death in service benefits are protected.
Council leader Ali Thomas said it provided a "clear pathway of support".
"Receiving a terminal diagnosis is an extremely difficult and distressing event for any individual and their loved ones," he said.
"I am proud that Neath Port Talbot council is leading the way for employers in Wales".
Neath Port Talbot council has become the first organisation in Wales to sign a charter to help terminally ill workers.