The move was announced hours after the firm downgraded China for the first time in almost three decades citing concerns about its ballooning debt and slowing economic growth.
Moody's decision came as China tries to clean up a toxic brew of unregulated and risky lending that for years has fuelled the economy's spectacular growth, though some analysts doubt Beijing's willingness to quit its debt addiction.
Beijing rejected the cut, saying Moody's had used an "inappropriate" method to assess the risks facing the economy.
The city's involvement in China's Belt and Road initiative also brings its economy and financial systems closer to the mainland, Moody's said.
"The downgrade in Hong Kong's rating reflects Moody's view that credit trends in China will continue to have a significant impact on Hong Kong's credit profile due to close and tightening economic, financial and political linkages with the mainland," it said in a statement.
Moody's cut Hong Kong from Aa1 to Aa2 but upped its outlook from negative to stable. Earlier it had downgraded China to A1 from Aa3 -- its first since 1989 months after the Tiananmen Square crackdown -- and also increased its outlook to stable from negative. It affirmed the Aa3 rating of Macau and upgraded its outlook to stable from negative.
"We are of the view that Moody's has overlooked the sound economic fundamentals, robust financial regulatory regime, resilient banking sector and strong fiscal position that Hong Kong has," Chan said.
He also said the Belt and Road initiative will help Hong Kong businesses enter new markets, which would benefit the city's economy, adding that China has also been a "key source of growth" for the global economy in recent years.
Hong Kong is semi-autonomous after it was handed back to China by Britain in 1997. It preserved its financial and judicial systems and enjoys liberties not seen on the mainland.
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