A rescue crew was sent to the 40m (131ft) tall Vilyuchinsky waterfall in the Kamchatka peninsula after four people became trapped on Thursday.
All were pulled from the ice, and a boy and his father were flown to hospital.
The regional government said the boy was in intensive care with serious injuries.
“The father is in hospital with the child,” Marina Volkova, the Kamchatka Territory’s deputy minister of health, said. “The day before, they flew in from Vladivostok.”
Separate groups of tourists left the scene safely and no further casualties were reported, Russia’s Emergencies Ministry told the Tass news agency.
The ministry said it had dispatched a helicopter with rescuers, medics and investigators to the site. The collapse of the ice is under investigation.
Russian media reports suggest an avalanche from a nearby volcano may have caused an ice block to collapse, but this has not been confirmed.
The Vilyuchinsky waterfall – also known as the Tsar Icicle – is formed by water melting off of a glacier on the slopes of the Vilyuchinsky volcano.
Cascading about 40m, the waterfall is one of the most popular tourist sites on the peninsula and attracts many visitors every year.