However, he added, some restrictions would remain, with a number of streets requiring a special pass.
The 10 sq km (four sq miles) area - home to government buildings and foreign embassies - was closed after the US-led invasion in 2003.
"The opening of the Green Zone is one of the measures we promised the people," a statement from Mr Abadi`s office said on Sunday.
"The prime minister opened the Green Zone to public passage and people in their vehicles came in droves," it added.
It is the latest in a series of steps by Mr Abadi to ease sectarian tensions and crack down on corruption.
Baghdad has seen weeks of protests over poor services and abuses of power.
The Green Zone - which lies on the west bank of the Tigris River - was declared a no-go area for the public in 2003.
The complex is protected by high concrete walls with barbed wire and a number of checkpoints.
Despite this, it has been targeted a number of times by militant groups.
Many Iraqis have long complained that the Green Zone has become a symbol of disconnect between the country`s top officials and ordinary citizens.
The opening comes a day after at least 18 people were killed in two car bomb attacks in Baghdad.
Dozens of people have been killed in bombings across the capital in recent months.
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