The transnational pipeline, which aims to deliver gas to Turkey and Europe, is on track to be completed in 2019 despite the hurdles it faced in past.
Russia's Gazprom said on Monday it had completed the sea portion of the first line of the TurkStream offshore gas pipeline across the Black Sea.
Gazprom, which plans to complete the pipeline in 2019, said in a statement that 1,161 km, of pipe had been laid since it began construction last year.
The second line, designed to ship gas to south European countries such as Greece, Bulgaria and Italy, will be laid in the July-September quarter of 2018, the company said.
Russian Energy Minister Alexander Novak said this month that Turkey's approval for Gazprom's onshore portion of the TurkStream pipeline's second line was still pending.
Moscow, which relies on oil and gas revenue, sees new pipelines to Turkey and Germany - TurkStream and Nord Stream 2 - as crucial to increasing its market share in Europe.
The project dates back to December 2014, but was put on hold after Turkey shot down a Russian jet which violated Turkish airspace near its border with Syria in November 2015.
The two sides agreed in October 2016 to restart the project during a visit to Istanbul by Russian President Vladimir Putin.
Turkey is the second biggest consumer of Russian natural gas after Germany.
It already imports about 30 billion cubic metres annually via two pipelines, the Blue Stream, which passes under the eastern Black Sea, and the Western Line through the Balkans.