Ngô Đình Cẩn (Vietnamese: [ŋo˧ ɗɨ̞̠n˦˩ kəŋ˦˩]; 1911 – 9 May 1964) was a younger brother and confidant of South Vietnam's first president, Ngô Đình Diệm, and an important member of the Diệm government. Diệm put Cẩn in charge of central Vietnam, stretching from Phan Thiết in the south to the border at the 17th parallel, with Cẩn ruling the region as a virtual dictator. Based in the former imperial capital of Huế, Cẩn operated private armies and secret police that controlled the central region and earned himself a reputation as the most oppressive of the Ngô brothers.

In his youth, Cẩn was a follower of the nationalist Phan Bội Châu. In the late 1940s and early 1950s, he worked to organise support for Diệm as various Vietnamese groups and international powers sought to stamp their authority over Vietnam. Cẩn, who succeeded in eliminating alternative nationalist opposition in central Vietnam, became the warlord of the region when his brother became president of the southern half of the partitioned nation in 1955. He became notorious for his involvement in smuggling and corruption, as well as his autocratic rule. Cẩn was regarded as an effective leader against the Viet Cong communist insurgency, which was much weaker in central Vietnam than in other parts of South Vietnam. His Popular Force militia was regarded by US officials in central Vietnam as a successful counter to the communists.Cẩn's influence began to wane after his elder brother Ngô Đình Thục was appointed the Roman Catholic Archbishop of Huế. Thục overshadowed Cẩn and aggressively promoted Catholicism, which led to the banning of the Buddhist flag in 1963 during Vesak, the celebration of the birthday of Gautama Buddha. Cẩn's forces opened fire on a crowd who were protesting the ban, killing nine and precipitating the Buddhist crisis. Ongoing demonstrations intensified throughout the summer as the regime responded with increased brutality, sparking the toppling of the Diem regime in a November 1963 coup. Cẩn had been offered asylum by the US Department of State, but ambassador Henry Cabot Lodge Jr. had CIA officer Lucien Conein arrest the fallen Ngô in Saigon. Cẩn was turned over to the military junta, which tried and executed him in 1964.