"The great leader Fidel Castro led his people's and his country's struggle against imperialism and hegemony for decades," Assad said in a message of condolences, AFP reported.
"His resistance became legendary and inspired leaders and people all over the world," he said in the letter addressed to Castro's brother Raul, who is president of Cuba.
"Cuba, a friendly country, was able thanks to its leaders to resist against the toughest sanctions and most unjust campaigns in our modern history," Assad said, referring to the US embargo on the island.
Fidel Castro, who built a communist state on the doorstep of the United States and defied US efforts to topple him for five decades, died on Friday at the age of 90.
He took power in a 1959 revolution and ruled Cuba for 49 years with a mix of charisma and iron will, creating a one-party state and becoming a central figure in the Cold War.
He was demonized by the United States and its allies but admired by many leftists around the world, especially socialist revolutionaries in Latin America and Africa.
Transforming Cuba from a playground for rich Americans into a symbol of resistance to Washington, Castro outlasted nine US presidents in power.
He fended off a CIA-backed invasion at the Bay of Pigs in 1961 as well as countless assassination attempts.