The DSP, a social-democratic oriented party, was registered on 14 November 1985 by Rahşan Ecevit, wife of Bülent Ecevit, as he was banned from political life after the 1980 coup d'état. In 1986 Bülent Ecevit addressed the DSP convention in Ankara, declaring his support for the party. The address landed him in court for allegedly violating the political bans. The DSP was unable, however, to achieve a substantial showing in the 1986 by-elections even though Ecevit, despite his ban, continued to campaign at the party's rallies as a "guest speaker". The political ban on Ecevit was lifted following a referendum in 1987. Later that year, Rahşan Ecevit handed over the rule of the party to her spouse. But the party failed to pass the 10% national threshold needed for a political party to have a seat in Parliament in the 1987 elections, prompting the Ecevits to step down from their positions in the party. In 1988, Necdet Karababa was elected as the new party leader. However, the next year, Ecevit was reelected as party chairman in the party convention. Two years later in 1991, the DSP received 10.75% of the votes in the elections allowing the party to have seven seats in the Parliament. This also meant the return of Ecevit to the Parliament after 11 years. It was a minor party until it won 76 parliamentary seats in the December 1995 elections. Again in 1995, the party started to suffer from inner conflicts after years of serenity. The inner conflicts in the party ended with the dismissal of Erdal Kesebir, MP for Edirne and three other DSP members. In 1997, the DSP became a partner of a three-way coalition government led by Mesut Yılmaz who at the time was the leader of the Motherland Party. Ecevit became the deputy prime minister. In 1998, the 55th government of Turkey was toppled by a censure motion. Ecevit received the mandate to form a new government. He founded a minority government of DSP in 1999 to carry the country to general elections.
Boosted by the capture of the leader of the PKK, Abdullah Öcalan, during his premiership, Ecevit and the DSP won 22,19% of the votes in the elections of April 1999 and took 136 of the 550 seats in the Turkish Parliament becoming the leading party. Its leader Bülent Ecevit became the Prime Minister of Turkey for the fifth time. The coalition government of DSP, MHP and ANAP pursued social and economic reforms and EU Harmonization reforms, and implemented changes to 34 articles of the Constitution. As Turkey suffered an economic crisis in 2000-2001, the government embarked on further economic reforms which included changes to the tender law, economic social council law, unemployment insurance, the restructuring of state banks, accreditation law, law on capital markets, and the law on industrial zones. The party staunchly opposed the invasion of Iraq by the US. The DSP was weakened by internal divisions in 2001, when Deputy Prime MinisterHüsamettin Özkan and several other leading DSP politicians and MPs founded the New Turkey Party. The year after, its government coalition partner MHP called for early elections, which were held on 3 November 2002. None of the three coalition parties were able to pass the 10% national threshold in those elections.
Before the 6th Party Congress on 25 July 2004, Bülent Ecevit announced that he would step down as party leader and leave active politics. During the congress, Zeki Sezer, deputy chairman since 2001, was elected as the new Chairman of the party. DSP entered the 2007 elections together with the Republican People's Party. The DSP-CHP alliance won 20.85% of the votes with the DSP getting 13 seats in Parliament. In 2008, together with many other politicians joining the DSP before the local elections of 2009, Mustafa Sarıgül, the Mayor of the Istanbul district of Şişli officially joined the party. DSP got only 2.75% of votes in the local elections of 2009. Zeki Sezer resigned and Masum Türker succeeded him. After Türker's succession, both Mustafa Sarıgül, the deputy of Şişli, and Rahşan Ecevit, the widow of Bülent Ecevit, resigned from the party and created short lived Democratic Left People's Party. Democratic Left People's Party dissolved itself after six months. After 2011 general elections, DSP received a major decline in Turkish politics, getting less than 0.25% votes in subsequent elections in 2011 and 2015. In 2015, Önder Aksakal succeeded Türker as party leader. The party lost eligibility to contest in 2018 elections, and they supported Nation Alliance.
Rahşan Ecevit and Bülent Ecevit are honorary presidents of DSP.
The term Demokratik Sol was created by Bülent Ecevit in his late CHP period, as a mix of social democracy, social liberalism, democratic socialism, secularism and Kemalism. With this term, the party established a link between universal values of the left and national-cultural heritage which makes the ideology "domestic". In other words, contemporary social democracy is made compatible with the conditions of Turkey. With Democratic Left, Ecevit made up a new synthesis by articulating new dimensions such as democratic socialism and social democracy to the nationalindependence movement or Kemalism. The symbol of the white dove was selected to symbolize the pacifism and social accordance. DSP approves of Turkish adhesion to the European Union, although it criticized the content of the Customs Union. During the premiership of Ecevit in 1999, Turkey became a candidate for the EU. DSP is against the idea of a united Cyprus state.