The move adds to a list of eight overseas activists deemed fugitives by authorities in July.
The five are Simon Cheng, Frances Hui, Joey Siu, Johnny Fok and Tony Choi, who are now based in various countries including the United States and the United Kingdom.
"All of them who have already fled overseas have continued to commit offences under the national security law that seriously endangered national security," Steve Li, an officer with the Hong Kong police's national security department, told reporters.
The five were accused of various offences under the law including incitement to secession and subversion as well as collusion with foreign governments or external forces.
Police issued wanted notices and rewards of HK$1 million ($A190,900) for each of the five.
China imposed the national security law on the Asian financial hub in 2020 after months of anti-government protests.
The law punishes acts including subversion, secession, collusion with foreign forces and terrorism with up to life in prison.
Joey Siu, an activist based in Washington DC and a US citizen, told Reuters that this was the first time a US citizen had been placed under such a warrant and this "demonstrated the extraterritorial reach of the national security law and the chilling effect that follows".
"I think democratic countries, especially the US, need to take a lead on addressing such transnational repression harassment tactics against activists like me," Siu said.
Another activist shrugged off the moves.
"If the government deems the quest for democracy and freedom a crime, we embrace the charges to reveal the genuine face of social justice, unyielding to authority," Simon Cheng, who is now based in the UK, said on social media platform X.
Hong Kong police said they also arrested two men and two women aged between 29 and 68 for allegedly providing financial assistance for activities endangering national security to two wanted activists, Nathan Law and Ted Hui.
These were the first such arrests on financial assistance grounds under the security legislation, which carries a maximum jail term of 10 years.
"We paid particular attention to the essence of the wanted persons, and tried to break their chain of financing by all means," Li said.
Li said the four had donated amounts of between HK$10,000-12,000 via an online crowdfunding platform to promote secession.
They are being detained for further investigation.
Another prominent activist, Agnes Chow, jumped bail and fled Hong Kong this month after what she called sustained pressure from authorities that had damaged her mental and physical health.
Chow said she was forced to travel to the southern Chinese city of Shenzhen where she was taken under police guard to a patriotic exhibition on China's achievements before authorities allowed her to travel to Canada for studies.
Li did not confirm or deny any of Chow's claims, including the forced China trip, but said police bail conditions could change according to different individuals and circumstances.
"If she has a chance to listen to this press conference, I hope she can seize the opportunity to return to Hong Kong, so that she won't become a fugitive," Li said.
