The Tigantourine  plant has been closed since the attack by al-Qaeda-linked gunmen who took hundreds of local and dozens of foreign workers hostage.
After four days the Algerian army ended the siege by storming the complex, but 29 insurgents and at least 37 hostages were killed.
The plant is now operating at about a third of capacity, reports said.
When in full operation, the plant produces about 9bn cu m of gas a year, or about 11% of the total produced by Algeria - a key supplier of gas to Europe.
Prime Minister Abdelmalek Sellal started up one of three gas streams at the plant, which is jointly run by BP, Algeria's state-owned Sonatrach and Norway's Statoil, state radio reported.
Sonatrach head Andelhamid Zerguine said he would call for armed guards to be deployed to help protect Algeria's remote desert energy installations.
The remote Algerian gas plant at the centre of a deadly hostage-taking last month has partially resumed production.