In what the organization called an "alarming surge," at least 1,634 people were executed last year, an increase of more than 50% on 2014
"The rise in executions last year is profoundly disturbing," said Salil Shetty, Amnesty International's Secretary General, in a statement.
"Not for the last 25 years have so many people been put to death by states around the world."
According to the rights group's annual report the dramatic increase was fueled by three nations -- Iran, Pakistan and Saudi Arabia -- who together were responsible for nearly 90% of executions in 2015.
A dubious distinction
"It is disturbing how the countries still clinging to execution, such Iran, Pakistan and Saudi Arabia, have all put people to death at unprecedented levels," Chiara Sangiorgio, Amnesty's death penalty expert, told CNN.
"Many governments' responses to evolving security threats is leading to the unraveling of human rights protections around the world."
Iran put 977 people to death in 2015, Amnesty says, over 200 more than in 2014, and mostly for "drug-related offences".
The NGO claims that at least four of these individuals were under 18 at the time of their crime, making their executions a "flagrant breach of international law".
In response to the Pakistan Taliban
attack on a school in Peshawar
that massacred children in December 2014, Pakistan lifted a seven year moratorium on the death penalty to allow executions for terrorism-related offences.
"Pakistan has made no secret of the fact that they linked the resumption of executions to the Peshawar attacks," said Sangiorgio.
In March 2015, the government confirmed it was changing its policy and executions were resuming for all capital crimes.
Pakistan executed 326 people in 2015, the highest number ever recorded by Amnesty in Pakistan.
Saudi Arabia has long faced international criticism for both the manner and the methods of its judicial killings.
In September 2015 a group of U.N. experts called for the Saudi monarch, King Salman bin Abdulaziz, to reconsider the high profile death sentence of Ali al-Nimr
, nephew of firebrand Shiite Cleric Sheikh Nimr al-Nimr.
Al-Nimr was 17 at the time of his arrest for Arab Spring-inspired protests, and faced execution by beheading and crucifixion
. His uncle was executed in January 2016
But the dubious distinction of executing the most prisoners rests with China
, according to Amnesty.
It estimates that China is the world's top executioner.
"Based on our own monitoring, we believe that thousands of executions continue to be carried out in China," Sangiorgio said to CNN.
Researchers can't calculate the precise number because China considers this information a state secret. But Amnesty researchers said that it's likely the number of executions has "diminished" as a result of "ongoing reforms."
'A year of extremes'
Amnesty said there were indications that the death penalty was declining in popularity in some countries.
"2015 was a year of extremes," said Shetty. "We saw some very disquieting developments but also developments that give cause for hope."
The U.S. carried out 28 executions, the lowest number since 1991.
And despite the increase in executions, fewer people were sentenced to death -- particularly in Nigeria, which saw a 74% drop.
Fiji, Madagascar, Republic of Congo and Suriname all abolished the death penalty in 2015 and the group says this means, for the first time, the majority of the world has banned execution as a form of punishment.
"Whatever the short-term setbacks, the long-term trend is still clear," said Shetty. "The world is moving away from the death penalty. The countries that still execute need to realize that they are on the wrong side of history."