Morales, who had accused the company of failing to meet commitments to expand phone network coverage, said employees would keep their jobs.
The Bolivian leader signed the deal at a ceremony in the capital La Paz, while riot police stood guard outside the headquarters of Entel in the city.
The announcement came as many countries across Latin America celebrated International Workers' Day, marked by union marches in support of labour movements.
The date also marks the first anniversary of Morales announcing new contracts that turned foreign energy firms into service providers for Bolivian state energy company YPFB.
"Today, May 1, 2008, we are consolidating the energy nationalisation," he said at a public ceremony.
"The Bolivian state has 50 per cent plus one share of the capitalist, or so-called capitalist, companies."
Referendum due
The president also signed an agreement to purchase a majority share in the gas production company Andina from its Spanish company, Repsol YPF.
Gas production company Chaco, pipeline company Transredes and German-Peruvian owned distribution company CLBH will also all be returned to state control.
Morales is a close ally of Hugo Chavez, the Venezuelan leader, who has also increased state control of companies in Venezuela and pledged to invest in Bolivia's energy sector.
Bolivia, the poorest country in South America, is a major supplier of natural gas to neighbouring Argentina and Brazil.
Morales's reforms have been criticised by some more hardline Bolivians.
His government's proposed rewriting of the constitution to increase indigenous peoples' rights, state control of the economy and land reform have been criticised in particular.
On Sunday voters in the country's Santa Cruz region are to vote in a referendum demanding autonomy from the central government.