The Tigers, who are already outlawed by the United States and the European Union, have been waging a drawn-out campaign for independence for Sri Lanka's minority Tamil community.
President Mahinda Rajapakse had used emergency laws to reimpose the proscription of the Liberation Tigers of Tamil Eelam (LTTE), government spokesman Maithripala Sirisena said.
"From today onwards, the LTTE is banned," said Sirisena, who is also the agriculture minister.
He said the president proposed bringing back the ban that was lifted in September 2002 to allow the Tigers to attend negotiations arranged by peace broker Norway.
Those talks eventually collapsed in 2006.
Government officials said the president had given the Tigers an ultimatum to release civilians inside rebel-held territory by the end of December or face a ban from the start of 2009.
The Tigers had not formally responded to the government's ultimatum.
Officials said the government had been considering since last Tuesday whether to