ADB president Takehiko Nakao said the money would be used to finance expressways, railways, power generation and education.
It covers 2016 through 2018 and compares with USD 1.5 billion allocated in the preceeding three years.
"The ADB is re-orienting its operations to meet the evolving needs of Sri Lanka as it moves to become an upper-middle-income country in the next few years," he said after talks with President Maithripala Sirisena and other leaders.
"Both tax revenue enhancement and effective management of public expenditure are urgent tasks for the government," he said.
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The new government has gone on a spending spree to deliver on election promises of higher wages and lower prices, increasing the budget deficit and causing concern over the balance of payments.
On Friday the central bank raised benchmark interest rates for the first time in nearly four years, by 50 basis points to 8.0 per cent.
The IMF, which sent a mission to review Sri Lanka's economy earlier this month, said it had warned the authorities they should make a "stronger effort" immediately to reduce the deficit.
Sri Lanka received USD 2.6 billion from the IMF in 2009 to boost its financial reserves, which had dropped below USD 1 billion at the height of fighting between Tamil Tiger rebels and government forces.