Many southern inhabitants are also ethnic Malays who often speak a different language from their more northern Thai neighbours.
In three of those southern provinces - Yala, Pattani and Songkhla – Muslims predominate. The former sultanate of Pattani, which covered these provinces, is seen by some as the cradle of Islam in the region.
The area was annexed by Thailand in 1902 as a buffer against British-ruled Malaya.
During the 1970s and early 1980s, however, an armed separatist movement sprang up seeking the region's reunification with Malaysia.
But change in government policy in the 1980s and 1990s, which aimed to strengthen national identity among all Thais and boost Muslim political representation, plus wider economic development, helped bring peace to the region.
More recently, Prime Minister Thaksin promised some $700 million in aid for the south, including the opening of an Islamic university in Narathiwat. But intermittent violence since late 2001 has prompted some to suggest the integration drive is failing.
Clashes in April 2004, in which more than 100 mostly young local Muslims died, represented a serious escalation of violence that began three months earlier with a raid on a weapons cache.
Locals blamed excessive force by the authorities for those deaths, most of which took place inside one mosque.
But some analysts have suggested the armed youths who battled security forces in April may have been encouraged by armed Islamist groups outside Thailand.
Hanbali, a senior commander in the Indonesia-based group Jemaah Islamiyah, which is alleged to have carried out bombings in Bali and Jakarta and was planning major attacks in Bangkok, was among five of the group's alleged members to be arrested in southern Thailand in 2003.
But others have blamed the government for stoking separatist sentiment with its heavy handed crackdown in early 2004.
Martial law has been imposed across the border provinces and Muslims still complain of discrimination and relative poverty.