Good Party (IP)'s Istanbul deputy Fatih Mehmet Şeker announced his resignation from the party on Friday.
The 43-year-old lawmaker submitted his resignation directly to the Supreme Court rather than the party administration and did not give an explanation for his decision.
"Our friend who says at every opportunity that he is a principled politician, if he is really is principled, should resign from parliament also," Party Chairwoman Meral Akşener said.
Earlier this month, another of IP's Istanbul deputies, Hayati Arkaz, resigned and promptly joined the Nationalist Movement Party (MHP).
The IP has been in distress since poor performance in the June 24 elections sparked a wave of resignations, including by key founding members.
Akşener gained 7.3 percent of the votes in the presidential elections, while the IP garnered 9.6 percent of the votes in the parliamentary elections, well below expectations. However, IP was able to send 44 deputies to Parliament thanks to the alliance formed with the main opposition Republican People's Party (CHP).