But he expressed irritation with Hillary Clinton, the US secretary of state, for saying that the US expected Cuba, which is facing its worst economic crisis since the 1990s, to make changes in exchange for better relations.
"I have to say, with all due respect to Mrs Clinton ... they didn't elect me president to restore capitalism in Cuba, nor to hand over the revolution," Castro said, who succeeded his brother Fidel Castro as president last year.
"We are ready to talk about everything, but ... not to negotiate our political and social system," he said.
"I was elected to defend, maintain and continue perfecting socialism, not destroy it," he added, prompting a long-standing ovation from assembly members, most of whom are members of the Communist Party.
Obama has said he wants to "recast" relations with Cuba and ease the 47-year-old US embargo by allowing Cuban-Americans to travel and send money freely to the island, 145km from Key West, Florida.
The US administration also reopened immigration talks with the Cuban government that were suspended by George Bush, the former US president.
But Obama and Clinton have said further improvements depend on Cuba making progress on human rights and political prisoners.
"It's true there has been a diminution of the aggression and anti-Cuban rhetoric on the part of the administration," Castro said.
But he noted the embargo remained and the ending of restrictions on Cuban-Americans had not yet been implemented.
Castro said Cuba's economic plan for 2010 would call on the country to have a "balance of payments, without deficit" and to put priority on producing products and services that bring in hard currency.
Cuba's economy grew by 0.8 per cent in the first half of 2009.