He had been hit on his head and legs, Colombo National hospital director Hector Weerasinghe said.
"He is conscious and is out of danger," doctor Weerasinghe said.
There was no claim of responsibility for the attack, but colleagues said Jayantha's media activism had been criticised by state authorities in recent weeks.
The New York-based Committee to Protect Journalists (CPJ) said the attack highlighted that Sri Lanka's independent media was still a target, despite the end of the war with Tamil Tiger rebels last month.
"The attack on Poddala Jayantha is part of a trend," said Bob Dietz, the CPJ's Asia programme coordinator.
"These attacks are a chilling reminder that journalists remain under attack in Sri Lanka even after the end of the government’s battle with Tamil sepa