It said his next hearing was scheduled for September 3, with him remaining in custody until then.
Serhii K. was arrested in a bungalow in the Italian province of Rimini on a European arrest warrant.
German investigations pointed to a Ukrainian cell of five men and one woman as the perpetrators of the pipeline blasts.
The cell members were believed to have chartered a yacht to carry out the attack, according to Der Spiegel magazine and other media.
German prosecutors said Serhii K. had used forged identity documents to hire the yacht, which departed from the German city Rostock to carry out the attacks.
When the Nord Stream explosions happened, Western powers were quick to point the finger at Russia, which was under sanctions for its invasion of Ukraine.
But Moscow in turn pointed the finger at Ukraine.
German prosecutors last year issued an arrest warrant for another Ukrainian man, named as Volodymyr Z., a diving instructor whose last known address was in Poland.
He was suspected of being one of the divers who planted the explosive devices in an operation that also involved a married couple who ran a diving school, according to public broadcaster ARD and other media.
The case is awkward for Germany and Ukraine, as Berlin has strongly backed Kyiv in its fight against Russia.
Ukrainian President Volodymyr Zelenskyy has said his Government knew nothing about any plan to blow up the pipelines.
– Agence France-Presse