Gene Pitney found dead in hotel
Pitney - who found fame with Twenty Four Hours From Tulsa - was pronounced dead at the Hilton hotel at 1000 BST.
He was on a UK tour and had shown no signs of illness. The cause of death is not yet known but is not suspicious.
His biggest success was in the 1960s and he enjoyed a 1989 revival with his chart-topping duet, Something's Gotten Hold of My Heart, with Marc Almond.
Almond said he was "saddened and shocked" at the news. "Together we had one of the biggest hits of 1989 with Something's Gotten Hold Of My Heart and it was an honour to have worked with him," he said.
"He was a great, unique singer of great, unique songs. Today is a sad day."
Mark Howes of his management company In Touch Music said the singer was found in his bed.
I've seen him quite a few times on this tour and he was fit and well. He said it was the best tour he had done for quite a few years
Mr Howes told BBC Wales that everyone had been shocked by the death and there had been no signs that he was ill.
"He did a good show last night at St David's Hall and it was wonderful," he said.
"I've seen him quite a few times on this tour and he was fit and well. He said it was the best tour he had done for quite a few years."
Pitney has continually toured over the last 40 years.
He received a glowing review for the South Wales Echo - "non-stop enthusiastic performance" during what proved to be his final concert in Cardiff.
Gene Pitney was staying in an executive suite at the Hilton
He had nine dates left on his 23-date UK tour and was due to appear at Bristol's Colston Hall on Wednesday.
Pitney's songs have been recorded by some of the world's biggest stars - Hello Mary Lou was released by Rick Nelson, Roy Orbison recorded Today's Teardrops as the B-side to his million-selling single, Blue Angel.
He is also credited with helping the Rolling Stones break the American market with his endorsement of the band.
Mick Jagger and Keith Richards wrote his hit That Girl Belongs to Yesterday which became the Stones duo's first composition to reach the American charts.
Pitney returned to the charts with Marc Almond in 1989
The son of a mill worker, Pitney said childhood ambitions of becoming a performer could not have been further from his mind.
He once recalled how his first solo performance at school degenerated into an embarrassing whimper as Pitney was petrified by the expectant audience.
Overcoming his nerves over the next few years, Pitney learned to play the guitar and piano and formed a schoolboy band.
It was during one of their gigs that his distinctive voice was discovered by the "the proverbial fat man with a cigar" who took him off to New York.
He is survived by his wife Lynne and three sons who live in his native Connecticut.