Islam is the most widely practised religion in Southeast Asia with approximately 242 million adherents in the region (about 42% of its population), with majorities in Brunei, Indonesia and Malaysia as well as parts of southern Thailand and parts of Mindanao in the Philippines respectively. Significant minorities are located in the other Southeast Asian states like Singapore and Cambodia. Most Muslims in Southeast Asia are Sunni and follow the Shafi'i school of fiqh, or religious law. It is the official religion in Malaysia and Brunei while it is one of the six recognised faiths in Indonesia.

Islam in Southeast Asia is heterogeneous and is manifested in many different ways. In some places in Southeast Asia, Islam is adapted to coexist syncretically with already-existent local traditions. Mysticism is a defining characteristic of Islam in Southeast Asia, with Sufism having a large regional following. Mystic forms of Islam fit in well with already established traditions. The adaptation of Islam to local traditions is seen as a positive thing by Muslims in Southeast Asia. Islam is part of everyday life for adherents in Southeast Asia and is not separated from "non-religious realms". Southeast Asia is the global region with the highest number of Muslims in the world, surpassing the Middle East and North Africa. Islam in Southeast Asia is neglected in Western study of Islam which centers around the Middle East.

Southeast Asian identity varies by regions that include Brunei, Cambodia, East Timor, Indonesia, Malaysia, Myanmar, the Philippines, Singapore, Thailand, and Vietnam. The heterogeneous nature of Southeast Asia combined with the widely varying practices and meanings of Islam suggests Islam in Southeast Asia has a multitude of variations in practice and belief. Islam in Southeast Asia has been adapted into varying local norms across Southeast Asia. The Abangan are the dominant group of Muslims in Indonesia. The practices of the Abangan are heavily influenced by mysticism and embody a unique form of Islamic practice that incorporates rituals inherited from their pre-Islamic ancestors.

Muslim traders have been visiting and residing in Southeast Asia as early as the 7th century, mainly through the port town of Barus, located on the west coast of Sumatra, modern day Indonesia. Dutch historian, J. C. Van Leur, estimated that Arab colonies were already formed in Barus by 674. Further evidence is the tombstone of Syekh Rukunuddin, who died in 672. These findings convinced the Indonesian government to establish Titik Nol (Zero Point) Monument in Barus in 2017, recognizing the city's role in the spread of Islam in Indonesia.

Arab accounts describe Arab ships reaching China by the 9th century, this shows that Muslim interaction would have reached Southeast Asia by this time. In 2017, a group of Japanese archaeologists, while working on a 9th century shipwreck in Quang Ngai, Champa of which is now Vietnam, discovered ceramic shards with inscriptions in Indic script, which refer to a place near what is now Hormuz, Iran. Inscriptions on some fragments end with a pentagram or a hexagram, which led Islamic historian Do Truong Giang to interpret it as the Seal of Solomon.

According to Edward H. Schafer, there are several passages in Chinese chronicles compiled during the Five Dynasties and Ten Kingdoms period (907–979) that mention some Islamic activities in the Champa Kingdom during the 10th century. The Chinese described that "the customs of the Chams are identical as those of the Arabs (Dashi or Tajik)," and "the Cham king, whenever he goes out, he wears a large shirt of Arab brocade or Sichuan brocade." An excerpt recorded in the Song Huiyao Jigao notes the practice of dhabīḥah (ritual slaughter) among the Chams:

[In Champa] There are also mountain cattles, but they cannot be used for ploughing. They are only killed in sacrifice to the spirits. When they are about to be slaughtered, a medium is instructed to offer prayers, which sound thus: Allahu Akbar. In translation, this means: "May he be early reborn."

Muslim traders along the main trade-route between Western Asia and the Far East are thought to have been responsible for the introduction of Islam to Southeast and East Asia. The religion was then further spread by Sufi orders and finally consolidated by the expansion of the territories of converted rulers and their communities.