Syria is back in the influential Arab League, more than a decade after being thrown out for its brutal repression of pro-democracy protests, which led to the ongoing civil war.
The move is further evidence of a thaw in relations between Damascus and other Arab governments.
Syria's readmittance comes ahead of a summit in Saudi Arabia later this month that President Bashar al-Assad may now attend.
The US and UK have criticised the move.
A state department spokesman said Syria did not deserve to be reinstated but that the US supported the Arab League's long-term objective of solving the crisis in Syria.
The UK's Minister of State Foreign Commonwealth & Development Affairs, Lord Ahmad, said the UK remained "opposed to engagement with the Assad regime" and that Mr Assad continued to "detain, torture and kill innocent Syrians".
In a statement, Syria's foreign ministry said it had received the League's decision "with great attention" and called for "greater Arab cooperation and partnership".
Foreign ministers from 13 of the 22-nation group's members were present at the meting in Cairo where the decision to readmit Syria was taken.
They stressed the need to end Syria's civil war and the resulting refugee and drug smuggling crises.
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