The Plaid Cymru AM Steffan Lewis has died at the age of 34.
Mr Lewis announced he had been diagnosed with bowel cancer in December 2017.
He was elected the regional AM for South Wales East in 2016, describing it as his "dream job".
He had been active in Plaid Cymru politics from an early age, addressing the party conference as a schoolboy in 1997.
Brought up in Crosskeys and Tredegar, Steffan attended Ysgol Gyfun Gwynllyw in Pontypool. He was Plaid Cymru's spokesperson on External Affairs, leading on Brexit.
More recently he's gained public attention for raising awareness about bowel cancer. He was diagnosed with the disease in late 2017. Despite his serious illness, Steffan Lewis remained active and prominent in the Assembly.
He said at the time of his diagnosis that he had received "incredible support" from family, friends and colleagues.
Flags outside the Senedd in Cardiff Bay have been lowered as a mark of respect for the Assembly Member.
His family said to lose him is the "greatest possible blow to our family".
He leaves behind his wife Shona and three-year-old son Celyn.
While receiving treatment for the illness, Steffan Lewis continued to work and raise awareness about living with a serious illness.
“Even though it was extremely difficult for him to talk about his illness while going through immense personal hardship, he was adamant that he had a duty to use his experience for the benefit of others", leader of Plaid Cymru Adam Price said.
Mr Price, said Steffan had an "astonishing breadth of talents".
A book of condolence is available in the Senedd for those wishing to pay tribute to Steffan Lewis.
His friend, fellow Plaid Cymru AM Rhun Ap Iorwerth said his "contribution has sadly been too short."
"When he had something to say you listened, he never had to raise his voice because people knew when Steff spoke he spoke, he was speaking because there was something worth saying."
Former Plaid Cymru leader Leanne Wood described Steffan Lewis as a "good friend".
First Minister Mark Drakeford said he was "one of the most decent and able politicians of his generation."