A block party or street party is a party in which many members of a single community congregate, either to observe an event of some importance or simply for mutual solidarity and enjoyment. The name comes from the form of the party, which often involves closing an entire city block to vehicle traffic or just a single street. Many times, there will be a celebration in the form of playing music, games, dance and activities with food such as popcorn machines and barbecues. When multiple streets become cordoned off, those are referred to as street fairs. These gatherings are especially popular during holidays or local celebrations and may range from informal get-togethers to large public events. The tradition has evolved globally, with local variations reflecting cultural, historical, and political contexts.

Block parties in many countries, particularly in urban areas, require permits and adherence to legal regulations such as street closures, noise control, public safety, and health guidelines.

As a form of activism street parties are festive and/or artistic efforts to reclaim roadways as public space by large groups of people. They were made known in Western Europe and North America by the actions of Reclaim the Streets, a widespread "dis-organization" dedicated to reclaiming public space from automobiles and consumerism.

In the 1980s, under martial law in Poland, street gatherings that resembled block parties became a form of peaceful protest. The Orange Alternative, a surrealist art movement, organized theatrical events in public spaces as a way to challenge the Communist government's control over daily life. These events often included costumes and spontaneous celebrations, attracting large crowds and momentarily turning public streets into spaces for freedom and community. While not like the traditional block parties seen in the West, these performances captured the essence of communal gathering that is central to many block party traditions worldwide.

In the UK, street parties are mainly known as private residents' events without wider neighbourhood publicity and have a festive cultural meaning, especially in England and the south of Wales. They have historically been held in the summer to commemorate major national events, such as VE Day or for royal events such as jubilees, with bunting dressing the street, and children having fun in the street. An estimated 10 million people took part in street parties in 1977 for the Queen's Silver Jubilee.

The tradition seems to have begun in the United Kingdom after World War I as residents' organised "peace teas" to celebrate the signing of the Treaty of Versailles in 1919.^{[AI-retrieved source]}

The tradition was boosted for the wedding of Prince William and Kate Middleton in April 2011 with about 1 million people joining in street parties. For the Queen's Diamond Jubilee in June 2012 about 2 million took part.

Some street parties are held annually or at any time for residents to meet their neighbours in a traffic-free street in a private street party. Some "street parties" are wider public events taking many forms with any social or cultural activity.