In a three-day session in Havana's Convention Center, the National Assembly of the People's government voted late Wednesday to establish that Cuba would "never return to capitalism."
The names of the deputies were called in alphabetical order and each one voted in the affirmative; 559 of 578 deputies were present.
Cuban President Fidel Castro, who took power in the 1959 revolution, presided.
"We need socialism more today than ever!" Castro said before the vote. "To guarantee the future, a strong ideological base is needed."
Those present rejected U.S. calls for a change in the country's government system, including multiparty elections.
"The homeland is sacred, the revolution is unconquerable and socialism is irrevocable," Vice President Carlos Lage said.
Last month, the U.S. government refused to lift the trade and travel restrictions on the country, which were first put in place when Castro allied himself with the Soviet Union.
Wednesday's vote also deals a blow to the Varela Project, a local effort to organize a referendum on reform, including freedom of expression and business ownership.