Raul, now 86, has been in power since 2006, when he took over after illness sidelined Fidel, who seized power in the revolution.
Diaz-Canel, who has spent years climbing the party ranks, was named the sole candidate for the presidency yesterday.
He will formally be confirmed as the country's leader at roughly 9:00 am (1300 GMT) Thursday -- the anniversary of the 1961 Bay of Pigs invasion when Fidel Castro's forces defeated 1,400 US-backed rebels seeking to overthrow him. Havana has long hailed the showdown as American imperialism's first great defeat in Latin America.
It is also the day before the new president's 58th birthday.
Diaz-Canel, who some say bears a passing resemblance to American actor Richard Gere, is a fan of The Beatles whose penchant for wearing jeans has set him apart in Havana's corridors of power.
Although he has advocated fewer restrictions on the press and a greater openness to the internet, he also has a ruthless streak, with harsh words for Cuba's dissidents and the United States.
Crucially, he will remain under the watchful eye of Castro, who will continue to serve as the head of Cuba's all-powerful Communist Party.
Once sworn in, Diaz-Canel will be tasked with pursuing reforms begun by Castro to open up Cuba's economy to small private entrepreneurs and reach a rapprochement with its Cold War arch-enemy, the United States.
In 2015, Havana and Washington renewed diplomatic ties, with then president Barack Obama making a historic visit to the island a year later. But, steps towards a normalization of ties have been severely curtailed since Donald Trump arrived in the White House last year.
Diaz-Canel will also inherit a youthful population hungry for change on the Caribbean island -- one of the world's last outposts of Communism since the collapse of the Soviet Union.
Cuba watchers and domestic analysts say he will favor continuity over change in the early days of his presidency, however.
As they began their momentous two-day meeting Wednesday, the 605 delegates of the National Assembly cast their votes for the new Council of State, which counts 31 members and whose head automatically becomes president.
"I have the honorable mission of putting forward comrade Miguel Mario Diaz-Canel Bermudez as president of the Council of State," commission president Gisela Duarte told assembly delegates, effectively ending any suspense over the identity of Cuba's new leader.
Delegates immediately broke into applause and the two men rose to share an embrace as Cuba's two other vice presidents, both veterans of the revolution, looked on from the row behind.
Thursday's session will simply formalize the vote.