The prime minister was expected to lead the party in October's presidential election. President Alassane Ouattara said the country was in mourning.
Ivory Coast Prime Minister Amadou Gon Coulibaly died suddenly on Wednesday aged 61.
The secretary to President Alassane Ouattara read a statement from the president on state TV confirming Coulibaly's death.
"Fellow compatriots, Ivory Coast is mourning. It is with deep pain that I announce to you that Prime Minister Amadou Gon Coulibaly has left us," he said.
Officials said Coulibaly was unwell during a ministerial meeting at the presidential palace in Abidjan and was rushed to a hospital where he passed away. It is currently unclear how Coulibaly died.
He had returned to the Ivory Coast last week after undergoing medical treatment for two months in France. Some have speculated that the medical treatment was related to his 2012 heart transplant.
'My younger brother, my son'
Ouattara's party has been in power since election violence ten years ago that left some 3,000 people dead after then-president Laurent Gbagbo refused to acknowledge his defeat in a run-off election.
Coulibaly served as the prime minister since January 2017 after serving as secretary general of the presidency for six years. Ouattara called Coulibaly "my younger brother, my son" during the statement.
"I salute the memory of a statesman, a man of great loyalty, dedication and love for his homeland," added Ouattara.
He was picked as the candidate for the ruling Rally of the Republican party in the upcoming elections in October. Coulibaly's death could throw the upcoming election into chaos. Ouattara is required to step down after finishing his second term in office.
kbd/msh (AFP, AP, Reuters)