Operation Lalang (Malay: Operasi Lalang, also called Ops Lalang; lalang is Malay for cogon grass and this was taken to mean "Operation Weeding") was a major crackdown undertaken by the Royal Malaysian Police from 27 October to 20 November 1987, ostensibly to prevent the occurrence of race riots in Malaysia. The operation saw the arrest of 106 to 119 people—political activists, opposition politicians, intellectuals, students, artists, scientists and others—who were detained without trial under the Internal Security Act (ISA). It was the second largest mass arrest in Malaysian history involving the ISA since the 13 May riots 18 years earlier. It also involved the revoking of the publishing licenses of two dailies, The Star and the Sin Chew Jit Poh and two weeklies, The Sunday Star and Watan.

The Malaysian government argued that racial tensions had reached a dangerous level within the country, forcing the government to arrest those responsible for stoking the tensions. The notion that race riots were imminent however is contested, and it is widely believed that the operation was designed to control the political opponents of the Prime Minister Mahathir Mohamad. The event, which was followed by the 1988 Malaysian constitutional crisis the following year, marked the beginning of a period of Mahathir's authoritarian rule.

A number of political developments and issues preceding the operation had caused mounting political and racial tensions in the country. According to the Malaysian government's white paper explaining the arrests, various groups had exploited the government's "liberal" and "tolerant" attitude and played up "sensitive issues", thereby creating racial tension in the country. This racial tension, the government said, forced the government to act "swiftly and firmly" to contain the situation.

The crackdown happened against a backdrop in late 1986 and 1987 of a split within UMNO into two opposing groups, commonly referred to as Team A led by Mahathir, and Team B led by Tengku Razaleigh Hamzah and Musa Hitam. Mahathir was challenged for the leadership and narrowly won, but faced a subsequent legal challenge on his win.

There were also attacks by the government on several non-governmental organizations (NGO) which were critical of various government policies. Mahathir called these "intellectual elites" as "tools of foreign powers" and saboteurs of democracy.

In addition, a number of race and religion-related issues had arisen which had a cumulative effect in raising ethnic tension. These included the switch to Malay language as a medium of instruction for optional courses in the departments of Chinese and Tamil studies at the University of Malaya, the use of Chinese characters in certain signboards, the deputy president of the Malaysian Chinese Association (MCA) Lee Kim Sai questioning use of the term pendatang (immigrants) that was seen as challenging Malay's bumiputra status, as well as rumours of forced conversion to or from Islam.

The immediate cause, however, was the Ministry of Education's decision to appoint some 100 senior assistants and supervisors to Chinese-medium primary schools. Concerns were raised by Chinese politicians and organizations that those appointed were Chinese who were not Chinese-educated, implying that students and parents might be forced to use English or Malay to communicate with the school personnel. Chinese educationalist groups contended that the move would limit the usage of Chinese in these schools.

On 11 October 1987, a 2,000-strong gathering was held by the United Chinese School Committees Association of Malaysia (UCSCAM, the association of Chinese school teachers and trustees, also known as Dong Jiao Zong) at the Hainanese Association Building beside the Thean Hou Temple in Kuala Lumpur. It was joined by prominent politicians from Chinese-based parties such as the MCA's Deputy President and Labour Minister Lee Kim Sai, the leader of Democratic Action Party (DAP) Lim Kit Siang, as well as representatives from GERAKAN and other parties. The meeting resolved to call a three-day boycott in Chinese schools if the government did not settle the appointments issue. The boycott was called off at the eleventh hour to allow time for the government to resolve the issue, nevertheless 57 schools went ahead with the strike on 15 October, either because they did not receive the notice of postponement, or they disagreed with the decision.