Castro for years was in charge of the country's security apparatus and its armed forces during the rule of his brother Fidel.
But after succeeding the ailing Cuban revolutionary leader, he embarked on a reformist course in order to secure communism's survival.
"This will be my last term," Castro, 81, told lawmakers after the National Assembly reelected him and named a new regime number two, Council of State Vice President Miguel Diaz-Canel.
Castro said he was "elected to defend, maintain and continue perfecting socialism -- not to destroy it," adding that his economic reforms will create "a less egalitarian society, but a fairer one."
Lacking his older brother's charisma, Raul Castro formally assumed Cuba's presidency in February 2008 and took the helm of the Communist Party in April 2011.
As president, he has maintained dialogue with Catholic leaders and eased repression against opposition leaders.
He also created conditions for private initiative in the economy, and lifted various bans -- like the prohibition against foreign travel -- that had made the lives of ordinary Cubans difficult.
Analysts describe Raul as tough but practical, quiet and low-key compared to the more charismatic and bombastic Fidel, a famous for a lawyerly grasp of detail and stamina, allowing him to hold forth for hours on end during public speeches.
Small in stature, bespectacled and with a trimmed mustache, Raul Castro does not share his older brother's proverbial eloquence.
But he is noted for a sense of humour and for his direct, concise manner, in sharp contrast to his more loquacious brother.
Pragmatic, and an excellent organiser, Raul Castro has already committed himself to ensuring the continuation of communism, while promising "structural and conceptual changes".
From the early days of the Cuban revolution, when he helped organise the insurgency from the rugged mountains of Sierra Maestra, Raul Castro has shaped Cuba's Revolutionary Armed Forces (FAR) into a complex institution.
Raul Castro was born on June 3, 1931, in Biran in the eastern province of Holguin, to a Spanish father, Angel Castro, and a Cuban mother, Lina Ruz.
A student of economics and a member of the Communist Youth Movement, Raul Castro was jailed alongside his brother following their 1953 assault on the Moncada military barracks, in what turned out to be a failed attempt to topple the dictatorship of Fulgencio Batista.
After being freed from jail, the two brothers went together to Mexico to prepare the landing of the boat "Granma", loaded with rebels, on December 2, 1956 in Cuba.
When the revolution triumphed in January 1959, Raul Castro became second-in-command.
As minister of defense, he oversaw an institution that existed largely to repel a potential US invasion.
While managing the military, he has faced considerable challenges, including the 1991 collapse of the Soviet Union.
Following the dissolution of Havana's main financial backer, Cuba's military formed a series of money-churning businesses, including a tourism corporation that runs a domestic airline, hotels, retail outlets and marinas.
Widowed following the 2007 death of his wife and communist revolutionary Vilma Espin, the father of four and grandfather of eight, is seen as the true head of the Castro clan, a role his brother never fully took on.
He is also said to rely on the counsel of his daughter Mariela, 44, a sex therapist who has long championed the rights of homosexuals.
He is described as a disciplined and energetic leader who enjoys mountain climbing and camping.
Some analysts and Cuban exiles, however, continue to portray him as an uncompromising hardliner.
Brian Latell, a former US Central Intelligence Agency analyst who wrote the book "After Fidel," describes him as a "Stalinist," whom he says is "as brutal or more brutal than Fidel Castro".