Gabonese President Omar Bongo Ondimba died at the age of 73, France 24 television reported on Sunday, according to Xinhua.
Bongo was reportedly receiving medical treatment in the Quiron Hospital in Spain's northeastern city Barcelona.
Bongo stayed away from public appearance after his wife died in March.
Gabon's first lady, the daughter of the Republic of Congo's President Denis Sassou Nguesso, passed away on March 14 in Rabat, the capital of Morocco.
President Bongo left for Europe early this month, sparking a spate of media speculation, saying the he was "seriously ill."
The Gabonese government said President Bongo "suffered a strong shock of emotional intensity following the death of his spouse after a long illness."
Gabonese Vice President Didjob Divungi Di Ndinge was put in charge of state affairs in Bongo's absence, reports said.
Bongo has been at the helm of the country for 42 years since he came to power in 1967, the longest serving president in Africa.
Gabon, a former French colony in the bight of Africa, forms part of the West Central Region of Africa. It has a population of 1.4 million and an area of 267,667 square km. Oil and timber are among the country's key industries.