The move would make Scotland’s streets safer and cleaner, and would encourage more people to walk and cycle, Mark Ruskell said.
A default speed limit of 20mph would also be cheaper than if local authorities were to try to implement their own schemes, he added.
His plans - contained in his Safer Streets Members’ Bill - are backed by several road safety and environmental organisations.
Mr Ruskell has already met with Transport Minister Humza Yousaf to discuss the Bill, and says he is confident he can attract the necessary cross-party support for the proposed legislation to proceed.
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Meanwhile, an opinion poll for the Greens also suggests public backing, with almost two-thirds of Scots supporting changing the default speed limit from 30 to 20mph in urban areas.
Some councils have already introduced 20mph zones through traffic regulation orders, but provision is patchy across the country and the process can be time consuming and costly, Mr Ruskell said.
In Fife for example, the 20mph limit applies to 4,804 streets, yet in Falkirk the figure is zero.
Research by Transform Scotland found an order can take between six and 12 months to complete, and cost between £1,500 and £3,000.
Edinburgh City Council recently introduced its own 20mph scheme at a cost of £2.2 million.
Mr Ruskell said his national scheme could be implemented in around two years, and would cost around £4.3 million.
“By bringing forward legislation I want to create safer streets so we reduce the risk for pedestrians and cyclists, especially children and the elderly,” he said.
“By cutting speeds in the streets where we live, shop and go to schools or day centres we can reduce deaths and injuries and boost public health with cleaner air.
“A wide range of interests from transport and health experts to environmental campaigners back the idea, as do the majority of Scots polled.
“I’d encourage anyone who wants safer streets in their community to respond to my consultation online and I look forward to discussing the aims of the Bill with various community groups around Scotland in the coming months.”
Irene Johnstone, head of British Lung Foundation Scotland, said: “There are around one million people here currently living with a lung disease.
“We need to explore all policy solutions to tackle this public health crisis.”
Stuart Hay, director of Living Streets Scotland, said: “We know that many communities across Scotland are concerned about the speed of vehicles in their streets.
“We also know that if speed is reduced then people of all ages are more likely to walk and cycle to school, to work and for local journeys.”
The public consultation will be open for three months.