The Social Democratic Party (SDP) is a former political party operating in the United States. They were formed in 1897 as a left-wing alternative to the mainstream parties. The following year the SDP managed early success when two members of the party were elected to the Massachusetts legislature.

In 1900 Eugene Debs stood as the party's presidential candidate in receiving some 97,000 votes. This was considerably more than the more established Socialist Labor Party.

In 1901 several members of the SDP, including Debs, left the party and established the Socialist Party of America. This party overshadowed the SDP who faded in political significance.

Note: this party is not to be confused with the later Social Democrats USA.