Fonseka, 65, joined the United National Party (UNP) and was made Kelaniya chief organiser of the party.
He obtained the party membership from party leader and Prime Minister Ranil Wickremesighe at the headquarters of the UNP, a party spokesman was quoted as saying by the Colombo Gazette.
The former army commander contested the last parliamentary election but failed to secure enough votes to enter the Parliament.
In February, Fonseka, who was earlier the leader of the Democratic Party, was given a vacant seat in Parliament.
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The move, however, had drawn angry reactions from Tamil political parties and human rights groups.
Fonseka has been accused by human rights groups of being involved in war crimes during the final stages of the nearly three-decades-long civil war.
The United Nations Human Rights Council (UNHRC) has asked for an investigation into human rights abuses by Fonseka's troops and the Liberation Tigers of Tamil Eelam (LTTE) during the final phase of the war that ended in 2009.
He was also sentenced to 30 months in jail after having mounted a failed bid to unseat Rajapaksa in his January 2010 re-election.
Fonseka, who was nearly assassinated by a LTTE suicide bomber in 2006, was pardoned by Sirisena days after he assumed office as the new president last year. All ranks and medals denied to Fonseka by the Rajapaksa regime along with his pension were restored.