Relations have been thawing between the neighbours with French backing
A signing ceremony has been held in Damascus establishing diplomatic relations for the first time between Syria and Lebanon.
A joint statement said Syria and Lebanon would respect each other's sovereignty and independence.
The two have had strained relations since the 2005 assassination of former Lebanese PM Rafik Hariri, which many Lebanese blamed on Syria.
Syria denied involvement; it withdrew its troops from Lebanon shortly after.
Syrian Foreign Minister Walid Muallem and his Lebanese counterpart Fawzi Salloukh signed a joint statement announcing "the launch of diplomatic relations between the Syrian Arab Republic and the Lebanese Republic effective today, 15 October 2008", Syria's national news agency said.
Lebanon's Prime Minister Fouad Siniora - a member of Mr Hariri's anti-Syrian Future movement - called it a "historic step forward on the road to affirm and solidify Lebanon's independence, sovereignty and free decision-making".
Anti-Syrian politicians in Lebanon and their Western backers have long called on their giant neighbour to establish official ties.
Syria's determination not to open an embassy in Beirut - as well as its stationing tens of thousands of troops in Lebanon until 2005, ostensibly as peacekeepers - was seen as a refusal to recognise Lebanese sovereignty and independence.
Relations have improved as pro- and anti-Syrian tendencies have tried to settle their differences in Lebanon, and France - the former colonial power - has encourage both countries to engage with each other.
During a groundbreaking visit to Damascus by Lebanese President Michel Suleiman in August, the two sides also agreed to tackle longstanding Lebanese demands to demarcate borders and investigate the question of missing Lebanese prisoners in Syria.
But tensions were raised again in September when Syria deployed 10,000 troops on the northern Lebanese border, prompting anti-Syria politicians in Beirut to raise the possibility of an invasion.
Correspondents say Damascus still has considerable influence over Lebanese affairs, much to the frustration of anti-Syrian politicians.
Syria's close ally, the Lebanese Hezbollah movement, which has Lebanon's most powerful military force, is now part of a national unity government and has the power of veto over its decisions.