Big-character posters (Chinese: 大字报; lit. 'big-character reports') are handwritten posters displaying large Chinese characters, usually mounted on walls in public spaces such as universities, factories, government departments, and sometimes directly on the streets. They are used as a means of protest, propaganda, and popular communication. A form of popular political writing, big-character posters do not have a fixed format or style, and can appear in the form of letter, slogan, poem, commentary, etc.

Though many different political parties around the world have used slogans and posters as propaganda, the most intense, extensive, and varied use of big-character posters was in China in various political campaigns associated with the Chinese Communist Party (CCP). Big-character posters were first used extensively in the Hundred Flowers Campaign, and they played an instrumental role in almost all the subsequent political campaigns, culminating in the Cultural Revolution. Though the right to write big-character posters was deleted from the Constitution of the People's Republic of China in 1980, people still occasionally write big-character posters to express their personal and political opinions.

Dazibao have been used in China since imperial times, but became more common when literacy rates rose after the 1911 Xinhai Revolution. They have also incorporated limited-circulation newspapers, excerpted press articles, and pamphlets intended for public display. Big-character posters can be seen as part of a long tradition of using writings to convey information and express dissent in public. Wall posters have been used in China to publicly announce royal edicts, pronouncements, and various orders since at least the Zhou dynasty (1046 BC – 771 BC), when posters were the only means of communication between the emperor and his people. Local governments also used such posters, for instance to announce news or describe the physiognomy of wanted criminals. Documentation of posters used to express political dissent can be found as early as the Han dynasty. Around 172 AD, a poster attacking the powerful eunuchs appeared on the gate of the Imperial Palace.

In the Republic of China, increased popular literacy enabled a more effective use of public posters as a form of political propaganda. Posters and other forms of public writings were frequently employed to express nationalist sentiment. During the May Fourth Movement in 1919, Peking University students were enraged by the Treaty of Versailles, which handed German-occupied territory in China to Japan after World War I and decided to hold a rally in protest. "Inflammatory notices" on the campus bulletin board announced their plan. When Sun Yat-sen died in 1925, and during the lying-in-state held in Central Park in Beijing, people spontaneously hung thousands of funeral scrolls around the park, which were often inscribed with a couplet expressing their grief and respect for the leader of the Xinhai revolution. Posters were also used extensively in 1925 during the May 30th Movement, for instance in urging patriotic citizens to stop using foreign products.

At the time, the Kuomintang (KMT, or the Chinese Nationalist Party) used slogans and posters in their political propaganda. Some early posters share the visual aesthetics of later big-character posters. For instance, a poster printed by the Political Department of the General Headquarters of the National Revolutionary Army led by KMT is dominated by big characters written in the center. The sentence read: "If the peasants want to plough their fields, they must help the revolutionary army." Compared to contemporary designs, which were often more flamboyant and distracting, this poster has a simple grotesque-style border that directs the reader's attention to the words. Although the poster was printed, the characters appear as forceful handwritings that convey a heightened sense of immediacy.

Inspired by the Soviet Union, the Chinese Communist Party (CCP) also used wall newspapers and posters in their propaganda campaign, as they could be easily produced and reproduced and were written in accessible language conducive to mass mobilization. It was commonly believed that big-character posters originated in Yan'an, the CCP headquarter during the Anti-Japanese War and the subsequent civil war. They not only disseminated news and communist ideas but were also used to purge party officials during the 1942 Rectification Movement. During this early use of big-character posters, both the target and the degree of criticism were strictly controlled by the CCP party units. However, occasionally, though rarely, people also used big-character posters to criticize the CCP. On March 23, 1942, Wang Shiwei, a 36-year-old pro-Communist journalist and writer, posted an essay titled "Two Reflections". Written in large characters, the essay criticized certain party leaders for repressing forms of political dissent. In the next week, several other posters similarly critical of the party also went up, which triggered intense debate among the party leadership. The party did not appreciate such public criticism of its operation, and the writers were punished. In particular, Wang was accused as a "Trotskyist spy" and beheaded in 1947. In 1945, during The 7th National Congress of the Chinese Communist Party, Mao Zedong reflected on Wang's big-character poster: "We were defeated by him. We acknowledged our defeat and worked hard at rectification." In 1957, when Mao Zedong had started to use big-character posters to mobilize the masses during the Hundred Flowers Campaign, he looked back to the Yan'an period in his talk at the supreme state conference: "A few big-character posters were written in the Yan'an period, but we didn't promote it. Why? I guess maybe we were a bit foolish back then."

In late 1956, Mao Zedong believed that internal contradictions within the socialist society and within the party leadership, such as issues of subjectivism, bureaucratism, and secretarianism, must be solved before they develop into serious antagonism that requires more violent and radical measures. To expose these contradictions, Mao was determined to create an open atmosphere in which people may freely air any constructive advice. In February 1957, he launched the Hundred Flowers Campaign, which attempted to mobilize the masses, especially non-party intellectuals, to voice their concerns and contend with each other.

However, as criticism started to target Mao's judgment and question whether China should be led by CCP and whether China should adopt the socialist path, Mao decided that it was enough. On May 15, 1957, he wrote an article "Things are starting to change", which was immediately passed around party cadres but not yet made public. In the article, Mao insisted that "In recent days the Rightists in the democratic parties and institutions of higher education have shown themselves to be most determined and most rabid... To date, the Rightists have yet to reach the climax of their attack, and they are going at it in high spirits... We shall let the Rightists run amuck for a time and let them reach their climax. The more they run amuck, the better for us... Why is such a torrent of reactionary, vicious statements being allowed to appear in the press? To let the people have some idea of these poisonous weeds and noxious fumes so as to have them uprooted or dispelled."