Mauritius joins India by refusing to send a premier to Sri Lanka, which is accused of widespread human rights abuses and the killing of tens of thousands of civilians during its 2009 defeat of Tamil Tiger rebels.
Canada is totally boycotting the summit.
"This is a decision taken by a sovereign Mauritius in the face of the absence of progress in Sri Lanka on the respect of human rights," Prime Minister Navin Chandra Ramgoolam told the Mauritian parliament.
He said that Mauritius, which will host the next Commonwealth Summit in 2015, believed that "human rights are more important than everything else". The prime minister also told AFP that the decision was taken because he had "closely followed the human rights situation" in Sri Lanka.
It will be the first time since the island's independence in 1968 that a Mauritian prime minister will be absent from a Commonwealth summit.
The Indian Ocean island will instead be represented in Colombo by its foreign minister, Arvin Boolell.
Ethnic Tamils make up an estimated 10 per cent of Mauritius' population. Menon Marday, a local Tamil community representative, welcomed the government's decision.
"We congratulate the prime minister, who has acted out of respect for human rights and against the oppression of the Tamil community in Sri Lanka, where the genocide continues," he told AFP.