There were more executions worldwide in 2015 than in any year since 1990 and almost 90 percent occurred in Iran, Pakistan and Saudi Arabia, according to Amnesty International, the UK-based rights group.
At least 1,634 people were executed last year, it said in a report published on Wednesday, adding that the actual number was probably significantly higher, given that there were no definitive numbers for China.
"The number of known executions rose by more than 50 percent compared with 2014 - this development is unsettling and alarming," said Oliver Hendrich, an expert on capital punishment at Amnesty International in Germany.
At least 977 people were executed in Iran last year, mostly for drug crimes, Amnesty said, while more than 320 death sentences were carried out in Pakistan and at least 158 people were executed in Saudi Arabia.
In the US, 28 people were executed last year, the lowest number since 1991, Amnesty said.
Countries that impose the death penalty are in the minority for the first time now, Amnesty said.
It said 102 countries had got rid of the death penalty for all crimes by the end of 2015, compared with 60 countries in 1996.
More than 20,000 people remain on death row across the world, according to Amnesty, despite the Republic of Congo, Fiji, Madagascar and Suriname becoming the latest of more than 100 nations to abolish the death penalty.
"Thankfully, countries that execute belong to a small and increasingly isolated minority," Hendrich said.
"The majority of states have turned their back on the death penalty, and in 2015 four more countries completely removed this barbaric punishment from the laws."