The TASS news agency said water levels in the sewers rose rapidly after a downpour and the victims were unable to escape to the surface.
"The death of five participants to the illegal excursion has been confirmed," the emergency services were cited as saying.
Video shared on social media showed investigators looking down a manhole in the capital, while divers searched the Moskva river.
Several tour operators offer trips into the vast tunnels of the capital's sewer system, some of which were constructed during the 19th century, though it was not clear if all the visits are permitted by authorities.
Daniil Davydov, described as an "urban explorer", told the RIA news agency that there were shelters in the tunnel where people could escape, but that nobody could be found.
"I hoped that maybe I would still be able to find some survivors there. There are two shelter points, but there was no one there," he said.
Police have opened a criminal investigation into the organiser of the tour, local media said.
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