Bolivian law already requires that salaried workers get a month's pay as a December bonus, so they will now get triple their pay for that month.
The leftist president said he made the surprise announcement because his government's goal is to reduce poverty and more equally distribute the wealth in one of South America's poorest nations.
"The economy is good and the country's growth should return to the workers with a double Christmas bonus," Morales said during a meeting with union leaders.
Bolivia's Federation of Private Companies vehemently objected, calling the decree "politicking" in a statement that said it "bodes terrible for future investment."
Morales, Bolivia's first indigenous president and a coca growers' union leader, first won office in December 2005 and is expected to seek re-election next year.
He has nationalized public services, including telecommunications and the electrical grid, and renegotiated natural resource extraction deals, but he has largely left the private sector intact.
Morales has, however, mandated annual wage increases to keep pace with inflation, which is now at 6.4 per cent annually.
Opposition Sen. Marcelo Antezana called the Christmas bonus decree "demagogic" and predicted it would aggravate inflation.
Bolivia is among countries that oblige all employers to pay workers an extra monthly wage in December. The new decree means Bolivia's salaried workers will get a third monthly wage for the month.
Those affected include some 300,000 state employees, who have doubled in number since Morales took office. Their median salary is a little more than $500 a month.
Bolivia's government is the single biggest employer in the country of 10 million people. Most Bolivians work in the informal economy and Morales has sought to extend a social safety net to them.
The bonus will be paid every year as long as Bolivia's gross domestic product grows by at least 4.5 per cent annually, the decree states.
The government says GDP is expected to grow about 6.7 per cent this year, thanks to record exports that are forecast to total $12 billion, half from natural gas sales and most of the rest from mining.
Associated Press writer Frank Bajak in Lima, Peru, contributed to this report.