A Panorama Deck vessel, which owned the Mermaid also, carried mourners onto the site of the disaster where they dropped a wreath and flowers into the river.
The Mermaid tourist boat collided with a larger Swiss river cruiser, the Viking Sigyn, in heavy rain on May 29, 2019, and sank in less than a minute.
Twenty-five South Korean tourists and two Hungarian crew members died, out of 35 people on board the vessel.
Zsolt Sogor, Panorama Deck's legal representative, said the anniversary opened "deep wounds" for many and that South Korean survivors would testify in an ongoing trial in September.
Prosecutors have charged Ukrainian national Yuri Chaplinsky, the captain of the Viking Sigyn, with one count of gross negligence leading to an accident with mass casualties.
They also charged him with failing to provide assistance to those aboard the Mermaid. Chaplinksy denies any wrongdoing.A memorial to the victims of the disaster will be unveiled on Monday (May 31).