Judge Sayeed Ahmed announced the decision Thursday in a Dhaka courtroom in the presence of seven defendants charged with murder in the death of Ahmed Rajib Haider.
The blogger was a campaigner for banning the Islamist Jamaat-e-Islami party, which opposed Bangladesh's independence from Pakistan in 1971.
One of the three who got the death sentence was tried in absentia.
The judge sentenced three others to 10 years in prison, another to five and another to three years.
Haider's father rejected the verdict saying all the defendants deserved the death sentence.
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