The African nation, scarred by the genocide of 1994, has become the 54th member of the association after a meeting of Commonwealth leaders in Trinidad and Tobago.
A Foreign Office spokesman said: "We strongly welcome the admittance of Rwanda as the 54th member of the Commonwealth.
"Rwanda has made progress towards meeting the Commonwealth's core values in areas of democratic process, rule of law, good governance, protection of human rights, and equality of opportunity and economic policies aimed at improving the welfare of the public."
Rwanda's accession to the Commonwealth had been widely anticipated, though some human rights groups questioned whether the country met the required standards of political freedom and human rights.
Rwanda has been rebuilding its economy after the 1994 genocide that killed about 800,000 people.
The last new member of the Commonwealth, which espouses democracy, good governance and respect for human rights among its key values, was Mozambique, the former Portuguese territory in Africa, which joined in 1995.
Commonwealth Secretariat spokesman Eduardo del Buey said Commonwealth Secretary-General Kamalesh Sharma had phoned Rwandan President Paul Kagame to tell him of the decision.
Rwanda's Minister of Information Louise Mushikiwabo said: "My government sees this accession as recognition of the tremendous progress this country has made in the last 15 years."
"Rwandans are ready to seize economic, political, cultural and other opportunities offered by the Commonwealth network."
Commonwealth members say the range and diversity of the large group, which brings together developed industrialised powers, developing countries and some of the world's smallest island states, creates an ideal forum in which to seek consensus on issues such as climate change and economic policy.
Before independence in 1962, Rwanda was under German, then Belgian rule. As a Francophone country, it forged close ties with France during Franois Mitterrand's presidency.
Mr Kagame had worked to bring his country into the English-speaking sphere in Africa after disagreements with France over events leading up to the 1994 Rwandan genocide. France had been accused of supporting the previous Rwandan government responsible for orchestrating the killing of ethnic Rwandan Tutsis and moderate Hutus.
Kigali cut diplomatic ties with France in 2006 after a French judge accused Mr Kagame and several officials of assassinating former president Juvenal Habyarimana.
Rwanda has since introduced English as the third official language, alongside French and the native Kinyarwanda tongue.
Analysts say Rwanda's bid to join the Commonwealth, after joining Burundi, Kenya, Tanzania and Uganda in the East African Community, is a deliberate step to distance itself from France.