The Social Democratic Party of New Zealand was an early left-wing political party. It existed only a short time before being amalgamated into the new Labour Party. During its period of existence, the party held two seats in Parliament.

The Social Democratic Party was founded in January 1913 at a so-called "Basis of Unity" Conference (often simply called the "Unity Conference"). This meeting drew together the most prominent left-wing groups in New Zealand, including both political parties and trade unions. The aim was to unite the fractious labour movement into a cohesive force. At the end of the Conference, most of the attendees agreed to merge into two new organisations — the new United Federation of Labour would co-ordinate the trade unions, while the two main political parties (the hard-line Socialist Party and the moderate United Labour Party) would merge to form the Social Democrats. Not all members of the United Labour Party accepted the plan, however, and some continued on under the same banner.

The Social Democrats gained a rapid boost when, shortly after their formation, Paddy Webb and James McCombs won by-elections and entered Parliament. Later the same year, however, a controversial strike broke out among groups of dockworkers and miners. Moderates in the union movement considered the strike ill-advised and dangerous, while radicals strongly supported it. The strike was heavily suppressed by the government of William Massey, and the United Federation of Labour was left broken and disorganised. The Social Democrats, still closely linked to the UFL, were plunged into disarray, with three of the party's leaders being jailed for their roles in the strike.

As a result of the chaos, the Social Democrats went into the 1914 elections with little in the way of planning. Co-operation with local labour organisations was sporadic, as was co-operation with the remnants of the United Labour Party. However, union anger at the government for its "heavy handed" response to the 1913 strikes was still strong, and the outbreak of World War I had also strengthened the labour vote. In the election, the Paddy Webb and James McCombs retained their seats under the Social Democratic banner. The remnants of the United Labour Party won three seats, and a labour-orientated independent was also successful. The six labour-aligned MPs worked together in Parliament despite being from different parties, with Alfred Hindmarsh of the ULP acting as de facto leader.

Two years later, in 1916, the close working relationship between the Social Democrats and the ULP remnant was formalised with a merger — the two officially came together as the Labour Party, the same organisation that survives today.