The Conservative Senedd member for South Wales East, Mohammad Asghar, has died at the age of 74 after being taken to hospital.
The Welsh Ambulance Service said it had received “reports of a medical emergency” on Tuesday morning.
Mr Asghar, known as Oscar in Cardiff Bay, represented the region for 13 years.
Welsh Conservative Senedd group leader Paul Davies called it a “devastating blow”.
“Our friend and colleague Mohammad Asghar has served the people of South Wales East in the Senedd with distinction for more than 13 years,” he said.
“I am sure that everyone will join me in sending condolences to his family.”
Wales’ First Minister Mark Drakeford said he was “saddened to hear” of his death.
“My thoughts are with his family and friends today,” he said.
“His presence in the Senedd will be missed.”