"He was on his way to work when he was shot... we heard that he had been shot in the chest, head and the abdomen," a colleague of Wickramatunga told the AFP news agency.
The Sunday Leader regularly lampoons politicians and has exposed several privatisation deals that eventually ended up being overturned by Sri Lanka's supreme court.
Minelle Fernandez, reporting for Al Jazeera from Colombo, said Wickramatunga had been attacked in the past. The newspaper also had its press burned in a previous attack, she said.
TV station attack
The shooting came just two days after more than a dozen men armed with guns and grenades stormed a private broadcaster's studios and destroyed much of its equipment.
The Maharaja Organisation's three TV stations and four radio channels were forced off air for several hours. There were no injuries reported following the raid.
The broadcaster had been criticised by state media for devoting too much airtime to a suicide attack by a Tamil Tiger separatist last week.
Reporters and media organisations have come under attack as the war between government forces and rebel Tamil Tigers has intensified.
Mahinda Rajapakse, Sri Lanka's president, had promised to defeat Tiger fighters by the end of last year.
He says government troops are close to eliminating the Liberation Tigers of Tamil Eelam (LTTE) after a series of battles that have forced the Tigers to retreat from former strongholds in the north of the island.
It is difficult to verify battle accounts and casualties as reporters are barred from the war zone.
The rebel Tigers have waged a 25-year war for independence for Sri Lanka's minority Tamil community, allegedly marginalised by successive governments controlled by the Sinhalese majority.
About 70,000 people are thought to have been killed since separatist fighting began in 1972.