Peninsular Malaysia, historically known as Peninsular Malaya or simply Malaya prior to 1963, and also known as West Malaysia and the Malaysian Peninsula, is the southern part of the Malay Peninsula in Mainland Southeast Asia along with the nearby islands that are part of Malaysia. Its area totals approximately 131,956 km² (50,948 sq mi). It makes up nearly 40% of the total area of Malaysia; the other 60% is on the island of Borneo, also known as East Malaysia.

The present-day land border with Thailand to the north originates from the Anglo–Siamese Treaty of 1909, while a maritime border with Singapore to the south was established following the independence of Singapore in 1965. To the west, across the Strait of Malacca, lies the island of Sumatra, and to the east, across the South China Sea, lie the Natuna Islands, both part of Indonesia. At its southern tip, across the Strait of Johor, lies the island country of Singapore. Most of Peninsular Malaysia's interior is forested, mountainous and rural, while the majority of the population and economic activity are concentrated along the coastal western half, where the country's major urban centres are located.

Peninsular Malaysia consists of 11 out of the 13 states, and two out of the three federal territories of Malaysia, which includes the national capital of Kuala Lumpur. The states are listed as the following:

Originally comprising the states and territories of the Federation of Malaya, the then Federation under the Malaysia Agreement merged with the Crown Colony of North Borneo, the Crown Colony of Sarawak and the self-governing State of Singapore to form the new Federation called Malaysia. The merger was initially proposed in order to reunify Singapore with its hinterland in the Federation as they were originally associated under British Malaya but later separated and were governed separately after the formation of the Malayan Union. Even when the Malayan Union transformed into the Federation of Malaya, Singapore was not a part of it. Although politically distinct, Malaya was then seen geographically as comprising the States of the Federation of Malaya in the Peninsula and Singapore. In order to facilitate the merger, the Borneo States (which initially also included Brunei) were brought in as well as it was believed that with the inclusion of the various ethnic groups in Borneo, the racial arithmetic would be offset such that the influx of ethnic Chinese from Singapore would not politically overwhelm Malaya, satisfying the Malay ultras.

Ultimately, both Malaysia and Singapore agreed that after a merger, Singapore would retain autonomy in labour, education and health, among others, unlike the other states in the Federation of Malaya. In exchange, Singapore received an underproportioned representation in the House of Representatives of Parliament. Singapore within Malaysia was seen as having a special status (similar to Northern Ireland in the United Kingdom) and was thus not grouped with the other non-autonomous states in the Peninsula. Although this arrangement was brief and Singapore was ultimately expelled from the Federation two years later in 1965, becoming a fully sovereign country, the Interpretation Act 1965 of the Parliament of Singapore still defines Malaya as comprising the States of Malaya and Singapore in a geographical sense. Today, the States of Malaya are colloquially referred to as Peninsular Malaysia and West Malaysia, excluding the Borneo States and Singapore. The term should also not be confused with the Malay Peninsula, which includes lands that are a part of Myanmar and Thailand.

Peninsular Malaysia (States of Malaya) comprises the states of Johor, Kedah, Kelantan, Malacca, Negeri Sembilan, Pahang, Penang, Perak, Perlis, Selangor, and Terengganu, as well as the federal territories of Kuala Lumpur and Putrajaya.

Malaya comprises Peninsular Malaysia and the Republic of Singapore.^{[citation needed]}

Malay Peninsula comprises the southern tip of Myanmar, Peninsular Malaysia, and Southern Thailand.