Reinstating Zelaya is "a fundamental step" for the Central American country to return to constitutional normality, the leaders said in a statement, calling the June coup that had forced Zelaya out of office "unacceptable."
The Honduras coup was Central America's first in 20 years.
The statement — released at the end of a three-day Iberoamerican summit in Estoril, Portugal — also said leaders at the annual meeting had analyzed Honduras' weekend election won by Porfirio Lobo, but they did not elaborate.
Summit participants discussed the global financial crisis and climate change, though the talks were overshadowed by efforts to reach a united stance on the Honduras crisis.
"Some will say (the statement) goes too far, others will say it doesn't go far enough," Portuguese Prime Minister Jose Socrates said.
Brazilian President Luis Inacio Lula da Silva said those behind the Honduran coup had no legitimacy to call elections.
"Honduras hasn't respected the most elementary principle of a return to democratic normality," he said before leaving Portugal on a state visit to Ukraine. "They could have done things normally: the president returns, calls elections and Honduras returns to normal."
Silva said accepting Honduras' current situation would provide encouragement to anyone planning a coup.
"We can't pretend nothing happened," Silva said. "If this state of affairs is allowed to remain, democracy will be at serious risk in Latin and Central America."
The summit statement also made an "energetic appeal" for Honduran authorities to ensure the safety of Zelaya, who has been sheltering in the Brazilian Embassy in Tegucigalpa for two months.