2001 Parliament attack convict Afzal Guru on Saturday was hanged after his final mercy plea was rejected by President Pranab Mukherjee.
Afzal Guru, a former fruit seller, was hanged at Tihar Jail at around 8am, home secretary RK Singh said.
Guru was found guilty of conspiring with and sheltering the militants who attacked the parliament in December 2001, an incident that brought nuclear-armed India and Pakistan to the brink of war.
He was also held guilty of being a member of the banned Islamist group Jaish-e-Mohammed, which fights against the country's rule in Kashmir, where a separatist conflict has claimed up to 100,000 lives.
Five armed rebels stormed parliament in New Delhi on December 13, 2001, killing eight police officers and a gardener before they were shot dead by security forces. A journalist wounded in the attack died months later.
Guru was initially convicted along with Shaukat Hussain, a former student at Delhi University and SAR Geelani, a New Delhi college teacher, who were also handed the death sentence, reserved for the "rarest of rare" cases.
Guru's wife, Afsan Guru, who was found guilty of not disclosing information to police, was also sentenced to five years in prison but had her conviction overturned on appeal.
The sole surviving gunman from the 2008 Mumbai attacks, Pakistani-born Ajmal Kasab, was executed on November 21, 2012.