Fonseka obtained the party membership from party leader and Prime Minister Ranil Wickremesighe at the headquarters of the UNP, a UNP spokesman said.
The former army commander contested the last Parliamentary election but failed to secure enough votes to enter Parliament.
In February Fonseka, who was earlier the leader of the Democratic Party, was given a vacant seat in Parliament.
Fonseka’s Democratic Party and the UNP had then reached an agreement to work together. Fonseka was also made a Minister in the Government.
The move however drew 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 war. (Colombo Gazette)