Some 800,000 people were killed in Rwanda's genocide in 1994
The Commonwealth has admitted Rwanda as its 54th member.
The African country was admitted at the Commonwealth Heads of Government Meeting in Trinidad and Tobago, a statement from the group said.
A Rwandan minister said the move showed his country's "tremendous progress" over the last 15 years.
The former colony of Germany and Belgium is the second country to be admitted without a British colonial past or constitutional link to Britain.
Mozambique is the only other Commonwealth member without historic UK ties.
It joined the organisation 14 years ago.
Information Minister Louise Mushikiwabo is quoted by the online edition of the Rwandan daily New Times as saying Rwanda was pleased by its admission.
"My government sees this accession as recognition of the tremendous progress this country has made in the last 15 years," she said
"Rwandans are ready to seize economic, political, cultural and other opportunities offered by the Commonwealth network."
A report in July by the Commonwealth Human Rights Initiative (CHRI) urged Rwanda to tackle a lack of political freedom and harassment of journalists before it was admitted.
It also articulated deep reservations over the country's human rights record.
"CHRI acknowledges that Rwanda has what appears to be a well-deserved reputation for governmental efficiency and for being less corrupt than a number of other countries - but its claims about the lack of corruption appear hollow when considering its complicity in the illicit economy of the region," the report said.
Rwanda expressed its desire to join the Commonwealth in 2008, despite its historic association with Francophone countries.
Relations between France and Rwanda have been poor for years.
A French judge implicated Rwanda's President Paul Kagame in the Rwandan genocide of 1994, an accusation which he rejected.
The Rwandan government has accused France of having backed the forces that went on to instigate the genocide.
The Rwandan New Times said Rwanda's bid to join the group was supported by Britain, Australia, Canada and India.
It also had the backing of Uganda, Kenya, Tanzania, South Africa and the host country Trinidad and Tobago, among others.