The Official Language Act (No. 33 of 1956), commonly referred to as the Sinhala Only Act, was an act passed in the Parliament of Ceylon in 1956. The act replaced English with Sinhala as the sole official language of Ceylon, with the exclusion of Tamil from the act.

At the time, Sinhala (also known as Sinhalese) was the language of Ceylon's majority Sinhalese people, who accounted for around 70% of the country's population. Tamil was the first language of Ceylon's three largest minority ethnic groups, the Indian Tamils, Sri Lankan Tamils and Moors, who together accounted for around 29% of the country's population.

The act was controversial as its supporters saw it as an attempt by a community that had just gained independence to distance themselves from their colonial masters, while its opponents viewed it as an attempt by the linguistic majority to oppress and assert dominance on minorities. The Act symbolizes the post-independent Sinhalese majority's determination to assert Ceylon's identity as a Sinhala Buddhist nation state, and for Tamils, it became a symbol of minority oppression and a justification for them to demand a separate nation-state, Tamil Eelam, which was a factor in the emergence of the decades-long Sri Lankan Civil War.

During the British colonial era, English was the official language in Ceylon (known as Sri Lanka since 1972). Until the passage of the Free Education Bill in 1944, education in the English language was the preserve of the Sri Lankan elite and the ordinary people had little knowledge of it. A disproportionate number of English language schools were established in Jaffna by the American Ceylon Mission, which provided English-language skills for the Tamil population in Jaffna. Thus, English-speaking Tamils held a higher percentage of coveted Ceylon Civil Service jobs, which required English fluency, than their share of the island's population:

By 1946, 33% of clerical jobs in Ceylon were held by Sri Lankan Tamils, although they were 11% of the country's population. However, the focus on percentages obscures the fact that the actual difference in number of jobs was quite small, for example, in 1948, Tamils accounted for 46% of government accountants and 40% of irrigation engineers, which was equivalent to 20 more Tamil accountants and 10 more Tamil engineers in proportion. The overall difference in number of government jobs was minute considering that the overall workforce was approximately 3 million people, most of whom were working in agriculture, petty manufacturing and trade.

After their election to the State Council of Ceylon in 1936, the Lanka Sama Samaja Party (LSSP) members N. M. Perera and Philip Gunawardena demanded the replacement of English as the official language by Sinhala and Tamil. In November 1936, a motion that "in the Municipal and Police Courts of the Island the proceedings should be in the vernacular" and that "entries in police stations should be recorded in the language in which they are originally stated" were passed by the State Council and referred to the legal secretary.^{[citation needed]}

In the 1940s, Sinhala political leaders were willing to support both Sinhala and Tamil as the official languages. For example, in 1944, both J. R. Jayewardene and S. W. R. D. Bandaranaike supported both languages getting official status, with Bandaranaike saying that he had "no personal objections to both languages being considered official languages, nor do I see any particular harm of danger or difficulty from this."

However, nothing was done about these matters, and English continued to be the official language until 1956.^{[citation needed]}