The ADB said that poor air quality had reached such a serious level that it was "jeopardising health and sustainable growth".
The loan is intended to cut the region's coal use, among other pollution issues.
China is the world's biggest polluter.
The loan will target Beijing city and surrounding areas including Hebie and Tianjin. The region is home to more than 100 million people and accounts for 10% of China's gross domestic product, the ADB said.
"Poor air quality has reached such a serious level that it is jeopardising health and sustainable growth in the capital region," said ADB's urban development specialist Satoshi Ishii.
"[Our] assistance will help reduce emissions and strengthen the environment regulatory framework and capacity of environmental monitoring and enforcement.
"Better air quality will benefit the economy and people's health, both in the region and beyond," he added.
The money from the ADB follows the latest World Climate Summit which is concluding in Paris this week.
The bank said in a statement the loan was expected to be accompanied by co-financing from Germany's KfW Development Bank of €150m ($164m; £108.2m).
The agreement follows Beijing's first "red alert" warning over pollution levels earlier this week.
It was issued late on Monday and lasted until Thursday, and was the first time that China had declared a red alert under the system, which was adopted a little over two years ago.
Schools in Beijing were closed, outdoor construction was halted, limits were placed on car use and some factories were ordered to stop operations.
Emissions from coal-powered industries, cars and heating systems, together with dust from construction sites all help create the dangerous smog.
When the red alert was declared, the US Embassy's air pollution monitor in Beijing said the intensity of the tiny particles known as PM 2.5 were at 291 micrograms per cubic metre.
A reading of 25 micrograms per cubic metre is recommended by The World Health Organization as the maximum safe level.
Despite large investments in renewable sources, China still depends on coal for more than 60% of its power.

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The Swiss, 79, is appealing to the Court of Arbitration for Sport after he was barred for a "disloyal payment" to suspended Uefa leader Michel Platini.
Both men deny any wrongdoing.
Speaking on the eve of Friday's vote to elect his successor as head of football's world governing body, Blatter said: "Enough is enough. I will always be a president."
Fifa's 207 member nations will choose a new president in Zurich, Switzerland.
In charge since 1998, Blatter had already announced he was quitting amid a corruption crisis.
Five candidates want to replace him - Sheikh Salman bin Ebrahim al-Khalifa, Gianni Infantino, Prince Ali bin al-Hussein, Tokyo Sexwale and Jerome Champagne.
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Blatter, under criminal investigation by Swiss authorities for corruption, told the New York Times he had been treated like "the last of the gangsters in the world" and said his ban was "not logical".
Blatter has not backed any of the candidates and said he will monitor the election in private.
Asked about the legacy of his 18-year tenure, Blatter claimed he will be remembered for making football loved across the world.
He pointed to the Paris attacks in November, in which bombs were let off outside the Stade de France as part of a co-ordinated attack across the French capital which killed 130 people and left more than 100 injured.
Blatter referred to the two explosions being "only outside the stadium" and added: "So far, all these outlaw groups, they have never attacked football because they know and they feel that football is for everybody.
"And no footballers have been killed while playing. I think football is something that puts people together that gives them hope."
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China is to receive a $300m (£198m) loan from the Asian Development Bank (ADB) to help it combat dangerous pollution levels in Beijing city and its surrounding area.
[NEXT_CONCEPT]
Outgoing Fifa president Sepp Blatter says he will not return to football - even if his six-year ban is overturned.