The trial is expected to last until February, the court in Hamburg announced.
The 50-year-old man was arrested in Italy last year and later extradited to Germany. Prosecutors say he used a false identity to rent the yacht used to carry out the attacks.
Leaks were discovered in September 2022 in the Nord Stream 1 and 2 gas pipelines, which run from Russia to Germany along the bottom of the Baltic Sea. It was later confirmed that the pipelines had been blown up.
Another Ukrainian suspect was arrested in August in Croatia, where he is reported to have been taking part in a film shoot about the attack. The man is described as a recreational diver.
A total of seven people are believed to have been involved in the sabotage.
