The 1955 system (Japanese: 55年体制, Hepburn: 55-nen Taisei), also known as the one-and-a-half party system, is a term used by scholars to describe the dominant-party system that has existed in Japan since 1955, in which the Liberal Democratic Party (LDP) has held by itself or in coalition with Komeito (from 1999 to 2025) a government nearly uninterrupted, with opposition parties largely incapable of forming significant or long lasting alternatives, other than for brief stints in 1993–1994 and 2009–2012. The terms 1955 system and the one-and-a-half party system are credited to Junnosuke Masumi [ja], who described the 1955 system as "a grand political dam into which the history of Japanese politics surge".

The years of Japan under the 1955 regime witnessed high economic growth, leading to the dominance of the ruling party in the Diet, with an undergirded tight connection between the bureaucracy and the business sector. Due to a series of LDP scandals and the 1992 burst of the Japanese asset price bubble, the LDP lost its majority in the House of Representatives in the 1993 general election, which initially signalled the end of the 1955 system. However, the left-wing Japan Socialist Party, the long-time opposition which finally gained a majority, would soon lose much of its support after it decided to form a coalition government with the arch-rival LDP just a year later, leading to the JSP being refounded as the SDP in 1996, and its coalition partner regaining power. The LDP briefly lost power again in 2009 to the now defunct Democratic Party of Japan before regaining it in 2012, retaining power up to the present day. Nevertheless, it lost its majority in the House of Representatives in the 2024 general election, and its majority in the House of Councillors in the 2025 election, forcing it to run a minority government for the first time.

In the modern day, Japanese politics often take form of a 1955-like system; although the formation of third parties and poles of opposition may be more frequent. Examples include Nippon Ishin no Kai, the Japan Restoration Party, and Kibō no Tō. These parties often take up a more ambivalent stance to the LDP, agreeing to cooperate on some policies. Numerous small groups also fade in and out of the system. The current main opposition to the LDP is the Constitutional Democratic Party of Japan, a re-foundation of the 2017 party which came from liberal Democratic Party splinters.

After World War II, Japan was controlled by the Supreme Commander for the Allied Powers (SCAP), which aimed at the eradication of militarism and the promotion of democratization in Japan. SCAP therefore issued a series of policies to arrest suspected war criminals which shuffled the political power in Japan. The power of right-wing parties declined in the immediate post war periods due to the purge. The Japan Progressive Party lost about 90% of the seats in the purge, while the Japan Liberal Party lost about 45%. Meanwhile, since many parties on the left were only legalized under the command of SCAP after the WW2, they were barely influenced by the purge. As a result, the Japan Socialist Party led by Tetsu Katayama won the first general election (1947) after the enforcement of the constitution of Japan.

Although at the time no formal regulations about how to form a coalition government existed, there was a consensus among the major parties that a coalition government should be formed to manage the post-war economic problems. Nonetheless, which parties would be included in the coalition took a long process of negotiations. Both a four-party coalition excluding the Japanese Communist Party (JCP) and a three-party coalition excluding both the JCP and the Liberal Party were suggested within the socialists. The Liberal party, led by anticommunist Shigeru Yoshida, showed great reluctance of joining the coalition. At a meeting between Katayama and Yoshida on May 19, 1947, the Liberal Party asked Katayama to "break off with the leftists" in return for its participation in the coalition. Since the Socialist Party had already claimed to officially cut ties with the Communist Party earlier, it refused such demand. The lengthy coalition-building process ended up with a government headed by the Socialist Party, the Democratic Party, and the National Cooperative Party.

The Katayama-led coalition government did not last more than a year due to opposition from both inside the Socialist Party and outside. In order to form a coalition, Katayama had to make concessions which departed from original policy proposals, which further facilitated the split between the leftist faction and the rightist faction within the party. At the same time, policies which Katayama did implement, such as nationalization of coal and mine production, drove the conservatives away.

After the failure of Katayama cabinet, voters lost confidence in the Socialist Party, which led to the takeover of government by Shigeru Yoshida. In the general election of 1949, the Democratic Liberal Party led by Yoshida won a majority in the House of representatives with 269 seats out of 466 seats, while the Socialist Party won only 48 seats. This was the first majority cabinet in post-war Japan.

Yoshida Shigeru organized five cabinets as a prime minister between 1946 and 1954. The diplomatic, economic, and security policies which Yoshida adopted when he was in power were altogether referred as the "Yoshida Doctrine". These policies remained influential even after him being voted out of the office by a no-confidence motion by the "Yoshida students" who followed his ideology.