"I had hoped to spotlight Beijing's deepening assault on the international efforts to uphold human rights," Roth said in the news release. "The refusal to let me enter Hong Kong vividly illustrates the problem."
Spokespeople for the Hong Kong Government did not immediately respond to requests for comment.
Human rights groups and others are concerned that Hong Kong's core values, protected under a framework known as "one country, two systems" that Beijing pledged to adhere to when it took over the former British colony in 1997, are quickly being eroded. These include free expression, freedom of the press and other basic rights that make Hong Kong significantly different from mainland China.
These fears have been heightened since June, when the Hong Kong Government tried to push through a law allowing extraditions to mainland China, sparking unrest that led to the arrest of about 7000 protesters. The extradition bill has since been withdrawn, but the pro-democracy demonstrations have continued, with a protest on Sunday in central Hong Kong that was attended by a few thousand.
In December, China said it would impose sanctions on US nongovernmental organisations, including Human Rights Watch and the National Endowment for Democracy, for their role in advocating for legislation that supported the protesters. A spokeswoman for the Foreign Ministry did not provide details at the time but said these organisations were being punished for their "odious behaviour" in Hong Kong.
Hong Kong and mainland China have entirely separate immigration systems under the "one country, two systems" framework. Roth, in a tweet, said authorities never specified why he was being denied entry despite his probing.
Earlier this month, a US photography professor who has covered the unrest in Hong Kong was denied entry into the territory. Immigration authorities did not specify the reason. In September, an American academic who testified on human rights issues before the US Congressional-Executive Commission on China in Washington was barred.
The Human Rights Watch report also planned to highlight curbs on human rights in Hong Kong. Roth will now launch the report in New York.