Shiite-majority Iran last week said it had hanged 20 "terrorist" Sunni prisoners on Tuesday convicted of carrying out a string of attacks against civilians and religious leaders in the country's western Kurdish region.
"Iran's mass execution of prisoners on August 2 at Rajai Shahr prison is a shameful low point in its human rights record," said Sarah Leah Whitson, Middle East director at HRW.
"With at least 230 executions since January 1, Iran is yet again the regional leader in executions but a laggard in implementing the so far illusory penal code reforms meant to bridge the gap with international standards," she said.
The New York-based rights group said "two lawyers who represented some of the men told (it) that their clients did not get a fair trial and that their due process rights had been violated".
It said rights groups believed the 20 were among 33 Sunni men, including possibly a minor, arrested in 2009 and 2010 and convicted of "enmity against God".
HRW said that recent changes to Iran's penal code required the judiciary to review and annul death sentences of people on that charge "if they had not personally used weapons in committing the crime".
Iran regularly hangs large-scale drug traffickers. Murder, rape, armed robbery and adultery are also capital offences in the Islamic republic.
Signup to our newsletter and follow us on Facebook and Twitter!
Those charged with "spreading corruption on Earth" and "enmity against God" can also be sentenced to death under Islamic sharia law, which has been in force in Iran since the 1979 revolution.
According to rights group Amnesty International, Iran was one of the world's top executioners in 2015 when it put 977 people to death, mostly on drug trafficking charges.
Amnesty does not include secretive China in its figures, but the number of executions in Iran exceeded both neighbouring Pakistan and regional rival Saudi Arabia.
Last week's execution, which was also criticised by the European Union and France, came as the EU had reportedly proposed talks with Iran on human rights.
Mohammad Javad Larijani, secretary general of Iran's High Council for Human Rights, said on Wednesday that Foreign Minister Mohammad Javad Zarif and top EU diplomat Federica Mogherini had already held preliminary discussions on the issue.
Larijani said Tehran was ready to discuss human rights but that "Westerners should not put themselves forward as role models".
Over two days in 2009, Iran hanged 44 convicted drug traffickers in one of the country's largest mass executions.
In 2013, it hanged 16 Sunnis in the eastern province of Zahedan, eight of whom were members of Sunni militant group Jundallah which waged a deadly insurgency in southeastern Iran for almost a decade.