Twenty-eight Hongkongers have been executed on the mainland since 2003, while the status of 19 others on death row is unknown, Amnesty International Hong Kong says.
Their crimes were mostly drug trafficking, murder, smuggling or tax evasion, the human rights group said.
Another five Hongkongers were slapped with the death penalty in Thailand, Malaysia and Vietnam, as were five Chinese nationals in Japan, although it is unknown if any Hongkonger was among them.
The Hong Kong government had failed to provide adequate support for locals who were so sentenced elsewhere, the group's Hong Kong director, Mabel Au Mei-po, said yesterday.
"The Immigration Department should know their situation best," Au said. "They could have been more active in the follow-up work to their cases instead of leaving the work to the local Chinese consulates."
On the mainland, another 28 Hongkongers were under suspended death sentences, the group revealed in its annual report on capital punishment.
Amnesty has long campaigned for worldwide abolition of the death penalty. The latest data was gathered from local and mainland media reports.
Excluding China, at least 778 people were executed last year, Amnesty said, compared with 682 in 2012.
China carried out the most executions last year, probably exceeding those in the rest of the world put together, it estimated. But Beijing's low transparency and lack of credible figures made it hard to say how far the estimated number reflected the actual situation, Au said.
But the mainland was inching towards eventually scrapping the penalty, after years of persuasion by foreign leaders and human rights groups, she said.
In 2011, the Chinese government took 13 offences off a list of 68 crimes punishable by death, she noted. Those offences included smuggling of items such as endangered animals and artefacts, and financial crimes.
Another welcome sign, Au said, was the top court's instruction to judges that year to limit the use of the death penalty. Amnesty urged people whose relatives were on death row outside Hong Kong to seek help.
The group could assist them, she said, citing the example of a Hongkonger in Malaysia who was allowed to see a doctor and meet Chinese embassy representatives after Amnesty intervened.
"Don't expect mercy will be shown if you [families of death-row inmates] keep silent, as what we saw in the past was the exact opposite," said Father Franco Mella, a spokesman for the group's joint committee for the abolition of the death penalty.