The League of Social Democrats (LSD) was a social democratic party in Hong Kong. Chaired by Chan Po-ying, wife of Leung Kwok-hung, it positioned itself as the radical wing of the pro-democracy camp and stressed on "street actions" and "parliamentary struggles".

Established in 2006 by a group of pro-grassroots left-leaning activists, the party opposed the perceived moderate and compromising approach of its pro-democratic allies Democratic Party and Civic Party and called for more aggressive tactics to achieve democracy. It often found itself at odds with other pan-democrats due to its confrontational and radical activism in the Legislative Council. The party first participated in the 2008 Legislative Council election and won over the 10 per cent of the popular vote and emerged as the new force with three seats.

In 2010, the League launched the "Five Constituencies Referendum" campaign to pressure the government to implement universal suffrage no later than 2012. The plan was joined by the Civics but rejected by the Democrats. The plan to target the Democratic Party in the following District Council election, as well as personal differences, led to a devastating factional struggles between the founding chairman Wong Yuk-man and his successor Andrew To, which resulted in the Wong's faction splitting from the party to form the People Power, leaving the party with only one legislator, Leung Kwok-hung.

The party suffered a sharp decline in the 2012 Legislative Council election but resumed a cooperative relationship with the People Power in the issues such as filibustering in the legislature and street activisms. Facing the rise of localism, the two parties formed an electoral coalition in the 2016 Legislative Council election which received seven per cent of the vote with Leung Kwok-hung being re-elected. Leung was subsequently disqualified over his oath-taking manner in 2017, leaving the party without any elected representation.

In the massive pro-democracy protests in 2019, its party member Jimmy Sham, who was also the convenor of the Civil Human Rights Front (CHRF), won one of two seats for the party in the following District Council election, and also won the nomination in the 2020 pro-democracy primaries where Leung Kwok-hung failed to secure a nomination. The party was dissolved in June 2025, amidst the political crackdown from the government.

The LSD was considered to be the "radical wing" of the pro-democracy camp by its political beliefs and tactics. It was formed by legislators, social activists and grassroots residents. It aims to be a "clear-cut opposition party" and defend the interests of the grassroots. It opposes the wealth inequality created by collusions between the government and corporations. It positions itself as a social democratic party and believes that a just society can be achieved by redistribution of wealth, economic interventionism and direct democracy.

Members of the party pioneered the use of theatrics and disruptive tactics in Hong Kong. Heckling and the throwing of projectiles have since become a frequent occurrence at Legislative Council and public meetings. Their members have been ejected from LegCo meetings on numerous occasions. At a Legislative council meeting on 15 October 2008, during the Policy Address given by then Chief Executive Donald Tsang, party chairman Wong Yuk-man threw a banana at Tsang in protest at the means test of "fruit money" (Old Age Allowance) for the elderly.

At the opening of an exhibition at the Museum of History on 2 March 2011, Steve Wong Chun-kit, member of the League of Social Democrats rushed at Donald Tsang. Protesters also threw cooked rice at Tsang, as a symbol of the plight of the poor, but missed him. Tsang said his chest was hit by the protestor and had a medical check at the Queen Mary Hospital afterwards. Tsang denounced the protest, saying that violence was unacceptable in Hong Kong, where civilised behaviour and the rule of law were fundamental values. However Leung Kwok-hung said he did not see any physical contact between Tsang and protesters. Steve Wong was arrested and released on bail.