The Sea Shepherd Conservation Society (SSCS) is a non-profit, marine conservation organization based in Friday Harbor on San Juan Island, Washington, in the United States. Sea Shepherd uses direct action tactics and information campaigns to protect marine life and inform the public of their plight. The organization was founded in 1977 under the name Earth Force Society by Paul Watson, a former member of Greenpeace. In 2008, Animal Planet began filming the weekly series Whale Wars based on the group's encounters with the Japanese whaling fleet in the Southern Ocean, which raised public awareness of the issue of illegal whaling and of the organization itself. Sea Shepherd has received widespread public support for its actions against illegal fishing, whaling, and seal hunting as well as support and funding from celebrities, such as musicians and TV personalities. Sea Shepherd currently operates nine vessels (see: Neptune's Navy). In addition to the organization's role of documenting and reporting violations of conservation laws, Sea Shepherd operations have utilized direct, non-lethal tactics including scuttling and disabling whaling vessels at harbor, intervening in Canadian and Namibian seal hunts, shining laser light at whalers, throwing bottles of foul-smelling butyric acid onto vessels at sea, boarding of whaling vessels to protest while at sea, and seizure and destruction of drift nets at sea. Sea Shepherd claims that their tactics are necessary to uncover and impede actions that violate international laws protecting wildlife, as the international community has shown itself unwilling or unable to stop species-endangering whaling and fishing practices. The tactics of Sea Shepherd have been opposed, even by some who denounce whaling, such as Greenpeace and some officials in the governments of Australia and New Zealand. Paul Watson and American members of Sea Shepherd are currently prohibited by US courts from approaching or harassing Japanese whalers, even if they are observed acting in defiance of international law, i.e., by killing whales in protected waters. The Japanese government, whose whaling industry is a leading target of the organization's efforts, have called Sea Shepherd eco-terrorists for impeding their research.However, in March 2014 the International Court of Justice ruled the Japanese whaling program in the Southern Ocean was not, as claimed, for scientific purposes, and ordered Japan to cease operations.

The predecessor organization of Sea Shepherd, the "Earth Force Society", was formed in 1977, after its founder, Paul Watson was ousted from the board of Greenpeace for disagreements over his direct action activism which clashed with their pacifist nature. Watson soon left Greenpeace. Initially without funding and with only a small group of supporters, in 1978 Watson convinced Cleveland Amory, head of the Fund for Animals to fund Watson's first vessel, the Sea Shepherd. The first direct action undertaken by Sea Shepherd was against Canadian seal hunting in the Gulf of St. Lawrence in March 1979. Also in 1979, the group made headlines when, for the first time, they rammed a pirate whaling vessel, the notorious Sierra. Such acts continued with Sea Shepherd claiming responsibility for damaging or sinking multiple whaling ships, through sabotage or ramming. The group has attempted to intervene against Russian, Spanish, Norwegian, Icelandic, Makah, Faroese, and Japanese whalers in multiple campaigns around the globe. Setting a pattern that the group would keep up in later years, the group managed to scuttle a Portuguese whaling vessel, though the first Sea Shepherd was impounded, and lost. Watson states he used the money gained from selling the story rights to fund his next vessel. After having spent the 1980s undertaking a variety of controversial and dangerous operations in support of various marine conservation aims, in the 1990s the group has been described as having undertaken a shift in their public attitude. Having previously argued primarily from an ethical viewpoint, from the 1990s, Watson's group now also started ascribing themselves law enforcement powers, using its interpretation of maritime and conservation law, to describe themselves as an anti-poaching agency. In some cases in the 2000s, they cooperated with official government efforts against maritime poaching, such as in Costa Rican waters, though the agreements often did not last long before conflict ensued. However, there is increasing co-operation with national governments in assisting anti-poaching activity in national territorial waters and marine reserves, including in 2016 with the Italian Coast guard in Sicily (Operation Siracusa), Mexican Navy (Operation Milagro) and the Coastguards of Gabon and São Tomé and Príncipe (Operation Albacore) which included having Gabonese military marines aboard. Starting in September 2010, Sea Shepherd Conservation Society has positioned a crew in Taiji, Japan to monitor and report their annual dolphin drive hunt.

Sea Shepherd is a non-governmental, non-profit environmental organization. In the United States it has a 501(c)(3) tax exempt status. Most of the organization's revenue—83.2 percent—is spent on its programs, while 16.7 percent of revenue is spent on administrative and fundraising. It is supported by private and corporate donations, lectures by Watson, internet advertising, and grants. The group is operated by volunteers and a small number of paid staff. Watson says he is committed to keeping his organization small, and does not believe in spending money on fundraising or recruitment. Sea Shepherd is governed by a board of directors, including Watson. The organization has several boards of advisers, each addressing an area of expertise. The Scientific, Technical, and Conservation Advisory Board includes Earth First! founder Dave Foreman and Horst Klienschmidt, a former deputy chair of the International Whaling Commission (2006). The Legal and Law Enforcement Advisory Board includes Ian Campbell, a former Australian Minister of the Environment and Heritage (2004–07) whom whaling groups had accused of having inappropriate and close ties with the organization. The Animal Welfare, Humane and Animal Rights Advisory Board included animal rights philosopher Tom Regan, until his death in 2017. The Media and Arts Advisory Board includes several major Hollywood stars. There is a Photography Advisory Board and a Financial and Management Advisory Board.

Sea Shepherd is governed by a board of directors, including Watson. The organization has several boards of advisers, each addressing an area of expertise. The Scientific, Technical, and Conservation Advisory Board includes Earth First! founder Dave Foreman and Horst Klienschmidt, a former deputy chair of the International Whaling Commission (2006). The Legal and Law Enforcement Advisory Board includes Ian Campbell, a former Australian Minister of the Environment and Heritage (2004–07) whom whaling groups had accused of having inappropriate and close ties with the organization. The Animal Welfare, Humane and Animal Rights Advisory Board included animal rights philosopher Tom Regan, until his death in 2017. The Media and Arts Advisory Board includes several major Hollywood stars. There is a Photography Advisory Board and a Financial and Management Advisory Board.