The Nord Stream project consortium said mid-April that it was closing the pipeline for two weeks to test and certify an integrated control system for the twin pipeline network.
The consortium said gas deliveries have resumed through the network.
"Nord Stream completed the process of adapting the control system to integrate the two lines into a single automated twin-pipeline gas transport system," it said in a statement.
Russia aims to diversify European transit options through Nord Stream, which runs from the shores of the Gulf of Finland through the Baltic Sea to Germany.
The consortium said it was entering "an intensive period" as it prepares the control system for the integration of the twin pipelines into a single network. Completion of the second line is expected by June.
Both lines, once fully operational, will transport around 1.9 trillion cubic feet of Russian gas each year to European consumers for at least 50 years.
Notable deaths of 2014 [PHOTOS]
EIA: Russia diversifying energy production