The question "who is the founder of Islam?" is one of the most searched questions about the religion, and the answer depends on whether you're asking from a historical/academic standpoint or from within Islamic theology itself — the two frameworks give somewhat different emphases.
From the Islamic theological perspective, Muslims do not consider Muhammad, peace be upon him, to be the "founder" of a new religion in the way that term is usually used. Instead, Islam teaches that it is the same core message of monotheism — submission to one God — that was revealed to every prophet throughout history, including Adam, Nuh (Noah), Ibrahim (Abraham), Musa (Moses), and Isa (Jesus). Muhammad (PBUH) is regarded as the final prophet in this chain, sent to restate and complete that message for all of humanity, rather than to invent something new. This is why the Quran describes Ibrahim as a "Muslim" in the sense of one who submitted to God, even though he lived many centuries before Muhammad (PBUH).
Historically, though, Muhammad (PBUH)' s role is central to how Islam as an organized religion, community, and set of practices came into being. Born in Mecca around 570 CE, he began receiving revelations, which Muslims believe came from Allah through the angel Jibreel (Gabriel), around the age of 40, in 610 CE. These revelations, delivered over roughly 23 years, were later compiled into the Quran, Islam's central religious text. Muhammad's (PBUH) teachings, along with the practices he demonstrated (later recorded as Hadith and Sunnah), formed the basis of Islamic law, worship, and community life as practiced today.
His early years of preaching in Mecca faced significant opposition from the polytheistic Meccan tribes, leading to the migration (Hijra) to Medina in 622 CE — an event so significant that it marks the starting point of the Islamic calendar. In Medina, the Muslim community grew into a organized society with Muhammad (PBUH) as both its religious and civic leader, and by the time of his death in 632 CE, much of the Arabian Peninsula had come under Islamic influence.
So when people ask who founded Islam, a historically accurate but theologically incomplete answer is "Muhammad (PBUH), in 7th-century Arabia." A more complete answer, reflecting how Muslims themselves understand it, is that Islam is presented as the continuation and completion of a monotheistic tradition going back to the very first humans, with Muhammad (PBUH) serving as the final messenger who delivered its last and complete form.
This distinction matters for understanding Islamic theology accurately: Muslims see Judaism, Christianity, and Islam as related through a shared lineage of prophets and a shared core belief in one God, even while differing significantly on later theological details, scripture, and practice. Recognizing both the historical role of Muhammad (PBUH) and the theological framework Muslims themselves use gives a fuller, more accurate picture than either perspective alone.
It's also useful to note what Islam's own sources say about this question directly. The Quran itself states that Muhammad(PBUH) was sent as a mercy to all the worlds and as the final prophet in the chain of messengers — a status Muslims refer to as "Khatam an-Nabiyyin," the Seal of the Prophets. This is why, within Islamic belief, there is no expectation of a future prophet after him, and why the Quran, as the revelation given specifically to him, is considered the final and complete scripture. Understanding this theological claim helps explain many other aspects of Islamic belief and practice that might otherwise seem puzzling from the outside.