A vulnerable man with learning difficulties was found stabbed in the stomach in the communal hallway to his flat, police said.
South Wales Police launched a murder investigation yesterday morning after officers discovered Peter Lewis, 68, at around 2.45am at his home on Claude Road, in the Roath area of Cardiff.
Police described the crime as "despicable" as they appealed for information about the attack.
Detective Superintendent Stuart McKenzie said: "Officers attended the scene where they found a man collapsed in the communal hallway suffering from an apparent stab wound to the stomach.
"Police officers attended to Mr Lewis while awaiting the the arrival of paramedics and Mr Lewis was then taken the University of Wales Hospital where tragically he died from his wounds."
A post mortem examination confirmed Mr Lewis died of a single stab wound.
Mr Lewis, whose family also lives in Cardiff, lived alone in the ground floor flat on the terraced residential street, which remains cordoned off by police today.
"There is no apparent motive," Det Supt McKenzie added. "We are obviously working through our lines of inquiry to establish why somebody would attack such a well respected, vulnerable, gentleman in the community."
Chief Superintendent Alun Thomas, the divisional commander for Cardiff, said Mr Lewis' family were "devastated" by the news and being supported by a family liaison officer.
"Our thoughts are with the family of Mr Lewis. They've got our full support during this terrible time," he said.
"He was a vulnerable man with learning difficulties. He was a happy man who was very well liked in the local community.
"He was well managed by the social services and he had good contact with the neighbourhood policing team, which makes this crime a despicable incident that is causing us to put every ounce of energy, activity and resource into it, in order to catch the culprit.
"It's understandable that the people of Cardiff and the people of Roath are very worried and shocked that such a serious incident has occurred locally.
"I want to reassure them that everything is being done to arrest this person."
A major incident room has been set up at Cardiff Central Police Station with over 50 officers working "around the clock".
Det Supt McKenzie renewed his appeal for information about a man who was seen in nearby Oakfield Street at around the time of the incident.
He is described as white, wearing a light-coloured hooded top and light coloured training shoes, and was running in the direction of Newport Road.
"Somebody out there knows who this man is, please come forward and help us track this man down," Det Supt McKenzie said.
"I would also like to point out that we are also interested in a similarly described man that was seen in the area in the hour prior to the incident that has been described by local neighbours as being potentially under the influence of alcohol or drugs."
Witnesses or anyone with information are asked to contact the Incident Room at Cardiff Central Police Station on 02920 571530 or Crimestoppers anonymously on 0800 555 111.