Saudi Arabia put to death 184 people in 2019 - a record number for the kingdom - despite a decline in executions worldwide, Amnesty International says.
The number of executions also doubled in Iraq to 100 last year, while Iran remained the second most prolific executioner after China, with 251.
However, global confirmed executions decreased for the fourth consecutive year to 657 - 5% less than in 2018.
It was the lowest recorded figure of the past decade, according to Amnesty.
The human rights group's tally does not include China, where the number of executions - believed to be in the thousands - remains a state secret.
It also notes that other countries, including Iran, North Korea and Vietnam, hide the full extent of their use of the death penalty by restricting access to information.
"The death penalty is an abhorrent and inhuman punishment; and there is no credible evidence that it deters crime more than prisons terms. A large majority of countries recognize this and it's encouraging to see that executions continue to fall worldwide," said Clare Algar, Amnesty's senior director for research.
"However, a small number of countries defied the global trend away from the death penalty by increasingly resorting to executions."
Saudi Arabia's growing use of the death penalty was an "alarming development", she added.
The kingdom executed 178 men and six women in 2019, just over half of whom were foreign nationals. The total was 149 in 2018.
The majority were convicted of drug-related offences and murder. But Amnesty documented what it called the "increased use of the death penalty as a political weapon against dissidents from the Shia Muslim minority".
In April 2019, there was a mass execution of 37 people. All but five were Shia men convicted on "terrorism" charges after trials that Amnesty said relied on confessions extracted through torture.
Ms Algar also said the large jump in executions in Iraq - from at least 52 in 2018 to at least 100 in 2019 - was "shocking".
The rise was largely due to the continued use of the death penalty against individuals accused of being members of the jihadist group Islamic State (IS).
In South Sudan, the authorities executed at least 11 people last year - the highest number recorded since the country's independence in 2011.
Yemen executed at least seven people in 2019, compared to at least four in 2018.
Bahrain and Bangladesh also resumed executions after one-year hiatuses.
Amnesty said several factors were mainly responsible for the global drop in recorded executions.
There were significant reductions in the number of confirmed executions in countries - such as Egypt, Japan and Singapore - that are strong adherents of the death penalty.
And for the second consecutive year, Iran executed fewer people than it had historically done, following amendments to its anti-narcotics law in 2017.
No executions were carried out in Afghanistan for the first time since 2010. There were also hiatuses in Taiwan and Thailand, both of which executed people in 2018.
Worldwide, 106 countries have abolished the death penalty in law for all crimes and 142 countries have abolished the death penalty in law or practice.