The ruling Movement Towards Socialism party succeeded in winning two-thirds of the seats in congress, including taking control of the senate from the conservative opposition, according to the exit polls.
If confirmed, the win will mean Morales will have the legislative numbers to pass laws without needing to negotiate with the opposition.
Official results are not expected until late Tuesday.
Bolivia's first indigenous president - a former llama herder who never attended high school - has a 60 per cent approval rating, much of it from the country's indigenous majority.
Since he took office in 2006, he has instituted quotas to give indigenous posts in the military and created a special school for aspiring diplomats with native backgrounds. He has also started three indigenous universities.
Morales also nationalised Bolivia's oil and gas sector in a move that helped lift the country's economy out of the red and build $8bn in reserves.
But he has also angered other Bolivians who have seen their landholdings decreased as part of the president's reforms.
Opponents say he has failed to increase output in the oil and gas sector, stamp out corruption in the state-run energy company and develop the natural gas industry.
|Morales supporters celebrating their candidate's sweeping victory [AFP]
Al Jazeera's Teresa Bo, reporting from the administrative capital, La Paz, said the pattern of voting, according to exit polls, indicated how divided the country still is, with the richest provinces continuing to oppose Morales.
Morales received only about 40 per cent of the votes in Santa Cruz, a key province that has opposed his presidency and sought autonomy from his central government, whereas he garnered more than 70 per cent in La Paz.
But Sunday's unofficial results indicate the opposition, which repeatedly challenged Morales in the early years of his presidency, is weakened even though it retains strong support in eastern regions, home to many of the country's natural resources and leading businesses.
Our correspondent also reported that while there were some complaints about missing ballots and tampering of ballot papers, according to election council officials, the Organisation of American States and the EU monitors were satisfied with the conduct of the election.
Morales re-election to a second term was made possible after he won a referendum that lifted the one-term limit for president.
The move mirrored similar moves by other Latin American leaders including Venezuela's Hugo Chavez and Rafael Correa of Ecuador.