Das was an activist of the Ganajagaran Mancha, the forum demanding a ban on Islamist parties and death penalty for convicted war criminals.
Mancha's Sylhet unit has called a half-day shutdown in the district for Wednesday in protest against the killing.
Das, 31, used to blog on 'Mukto-mona’, a website propagating rationalism and opposing fundamentalism started by Roy, who was killed in February in Dhaka.
Airport Police Station OC Gausul Hossain said the murder took place at Bankalaparha on Tuesday morning.
“Four armed assailants attacked him when he was going to the city in a rickshaw,” Hossain said.
Das was rushed to the hospital, where he succumbed to his wounds.
A Twitter profile claiming to be of extremist group ‘Ansar Bangla 8’ said the al-Qaeda in Indian Sub-continent (AQIS) claimed responsibility for the killing.
A purported AQIS message warned of killing of more “atheist bloggers”.
"We don't forget and we will NOT forget others who insult our beloved Prophet Muhammad (PBUH) and Allah (S:)," the message read.
Bangladeshi investigators say they are considering the terrorist organisation’s involvement in the killings.
Third blogger to die in three months
Secular writers have long been target of religious extremists in secular Bangladesh.
In 2004, machete-wielding men attacked Humayun Azad, a prolific writer, on the Dhaka University campus.
Rajshahi University teacher AKM Shafiul Islam died in a similar attack last November when returning home near the university.
A radical group ‘Ansar Bangla 2’ claimed responsibility on a Facebook post, saying he had been killed as he prohibited his department’s female students from wearing veils.
Tuesday’s victim, Das, who edited a science magazine ‘Jukti’ (logic) and authored several books, was a regular blogger and wrote in bdnews24.com blogs. He got Mukto-mona’s rationalist award in 2006.
He is the third blogger to be killed in three months.
In February, Avijit Roy was hacked to death near Dhaka University’s TSC. A month later, another blogger Oyasiqur Rahman was killed in a similar fashion in the capital as violence against secularists continued.
Earlier this month, al-Qaeda's Indian offshoot claimed responsibilities for the killings of secular bloggers in Bangladesh whom it called 'blasphemers'.
All three killed were involved with Ganajagaran Mancha.
Radical group Ansar Bangla claimed responsibility for the deaths.
Two years ago, Mancha activist Ahmed Rajib Haider, another blogger, was hacked to death near his Dhaka home.
The Mancha took out a march at Shahbagh on Tuesday, blaming the government’s “failure” to take action for the killings.
“The latest killing proves the government’s indifference and failure to tackle militancy and terrorism,” Mancha spokesperson Imran H Sarker said in a statement.
“Secular Blogger #AnantaBijoyDas was hacked to death today for his writing & voice against injustice,” he tweeted. “No Justice, impunity!”