[13] It was previously known as the Japan Socialist Party (日本社会党, Nihon Shakaitō, abbreviated to JSP in English).

Five leftist legislators who did not join the SDP formed the New Socialist Party, which lost all its seats in the following election.

[15] In 1995, the former Japan Socialist Party (JSP) was in a deep crisis, facing criticism for entering a coalition with its long-time rival, the LDP and for core policy changes.

Under Murayama's successor Ryūtarō Hashimoto (LDP), the SDP remained part of the ruling coalition.

After its electoral defeat in the 1996 general election, when it lost another 15 of its remaining 30 seats in the lower house, the SDP left the ruling coalition, which it had entered as the second-largest force in Japanese politics, as a minor party.

In the 2009 Japanese general election, the DPJ made large gains and the SDP maintained its base of 7 seats in the, becoming a junior partner in a new government coalition; however, disagreements over the issue of the Futenma base led to the sacking of Fukushima from the cabinet on 28 May and the SDP subsequently voted to leave the ruling coalition.

[20] During the nomination period of the 2016 Japanese House of Councillors election, the party signed an agreement with the Democratic, Communist and People's Life parties to field a jointly-endorsed candidate in each of the 32 districts in which only one seat is contested, thereby uniting in an attempt to take control of the House from the LDP/Komeito coalition.

[21] The party had two Councillors up for re-election and fielded a total of 11 candidates in the election, 4 in single and multi-member districts and 7 in the 48-seat national proportional representation block.