The Peoples' Democratic Party (HDP) lost nearly a million votes since the June general election, according to the initial results of Sunday's election rerun.
The party received 5,059,392 votes Sunday compared to 6,057,500 on June 7.
With 99.18 percent of the vote counted, the Justice and Development (AK) Party took 49.38 percent, which translated to 316 parliamentary seats in the 550-seat Grand National Assembly.
The Republican People's Party (CHP) received 25.41 percent (134 seats); the Nationalist Movement Party (MHP) got 11.94 percent votes (41 seats); and the HDP secured 10.69 percent (59 seats).
During the June election, the HDP passed the 10 percent threshold for the first time with 13.07 percent of the vote to take 80 seats.
In Istanbul, the HDP's number of seats fell from 10 to seven. It also came second to the AK Party in northeastern Kars and Ardahan provinces, having come first in June.
Turnout was 85.69 percent across the country.
The preliminary results were released after Turkey's Supreme Election Board lifted a broadcast ban at 7 p.m. local time (1600GMT).
More than 54 million Turks were eligible to vote in a poll to elect 550 deputies to the assembly.