In February Abdul Kader Mullah, of the Jamaat-e-Islami party, was convicted by a special tribunal of crimes during the 1971 war of independence with Pakistan.
The life sentence imposed at the time sparked angry street protests.
His supporters said the charges were politically motivated but his opponents said that sentence was too lenient.
When the war crimes tribunal sentenced Mullah in February this year, it triggered a wave of protests led by youth activists. Thousands of people had gathered at a busy traffic junction in Dhaka demanding the death penalty. They said the sentencing was too lenient.
The protests, which came to be known as Shahbag movement, spread to other parts of Bangladesh, rallying secularists across the country. The anger on the streets forced the government to amend a law to allow the state to appeal against any verdict or sentence passed by the tribunal – but this also drew criticism from rights groups
Security was tight and the five-judge bench delivered the verdict in a packed court room, saying Mullah deserved the death penalty in connection with the killing of a family during a Pakistani army crackdown in Dhaka in 1971.
Thousands staged protests around the country demanding the death penalty for Mullah, who had been convicted of charges which included overseeing massacres during the bloody struggle for independence.
The protests prompted parliament to amend a law which allowed the state to appeal against any verdict reached by the war crimes tribunal.
This meant that alongside Mullah’s appeal against his verdict the Supreme Court also heard an appeal from the government which wanted a tougher sentence handed down.
Prosecutor Ziad Al Malum told AFP news agency that the decision to increase the sentence was approved by four to one at the court.
Local media reports cite Mullah’s lawyers saying they will petition for this harsher sentence to be reviewed, but the attorney general has said there can be no appeal against a Supreme Court verdict.(BBC)