The consortium, led by France's Alstom Conglomerate and also including Spain's Acciona and Turkey's Gulermak, will construct a 15 kilometre (nine mile) extension to the site of the fair, a statement said.
Alstom will supply 50 trains, 15 of them for the new extension and 35 to upgrade existing services, the statement said.
Ten international consortiums bid for the project, it said.
"Negotiations were made with two of them in the final stage," it added.
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"Both of them are competent but financially we find it cheaper for us to go with Alstom," he said.
The government has already provided USD 409 million for the first six months, Tayer said, adding that the RTA has different options for funding the remainder of the project.
The consortium, dubbed Expolink, will begin work on the project in the final quarter of 2016, Tayer told a briefing.
Services should begin on May 20, 2020, five months ahead of the trade fair, he said.
The Dubai Metro, which transformed transportation in the Gulf city state when it opened in September 2009, was built by a consortium led by Japan's Mitsubishi.
Alstom chief Henri Poupart-Lafarge said his consortium was "proud to have been chosen," describing the contract as a "new major award for Alstom in the UAE".
"We will bring the most innovative and cutting-edge technologies to Route 2020 project and continue to be reliable partners of RTA," he told reporters.
"We are the world leaders in integrated urban systems. We equally have strong experience here in Dubai... Our international experience and our local knowledge here and in the Gulf allowed us to make a good offer," he later told AFP.
The Dubai Metro has two lines, a 52 kilometre (32 mile) Red Line and a 23 kilometre (14 mile) Green Line.
The new project will extend the Red Line from Nakheel Harbour & Tower station to the Expo 2020 site. It will feature 11.8 kilometres (7.3 miles) of elevated railway and 3.2 kilometres (two miles) of underground.