The 47-year-old Englishman took over at Afghanistan in November and won one, drew two and lost three of his six games, the highlight being a 2-1 away win at India in World Cup qualifying.
Westwood also oversaw a 0-0 draw with Asian champions Qatar as Afghanistan narrowly missed out on reaching the next stage of qualifying for the 2026 World Cup.
"It's a very privileged position, I feel honoured and proud," said Westwood as he was presented as the new boss of Hong Kong, who are ranked 159 in the world.
"I am sure together we can get some success in the future," added Westwood, whose managerial career has also included club jobs in India and Malaysia.
"The target is to climb up the rankings and I think we can do that quite quickly.
"The long-term aim is to qualify for the Asian Cup, which will also signify success."
Westwood succeeds the Norwegian Jorn Andersen, who quit in May having taken Hong Kong to their first Asian Cup in 56 years at the start of the year.
