Democratic Left (DIMAR) MP Spyros Lykoudis resigned on Tuesday, reducing the number of seats held by the small leftist party in the 300-seat House to 10.
In a letter addressed to Parliamentary speaker Evangelos Meimarakis, Lykoudis said he would continue as an independent.
His formal resignation came after a radio interview on Monday in which Lykoudis had expressed his intention to quit due to irreconcilable differences with the party's central committee.
DIMAR had 17 seats in Parliament after winning 6.3 percent of the vote in the general elections of June 2012 and joining a three-party governing coalition.
It saw its popularity drop to 1.2 percent in European elections in May after quitting the coalition over the closure of state broadcaster ERT in June last year.