Abdul Karim al-Khiwani, who was a member of the Revolutionary Committee of the Huthis that control Sanaa, was shot close to his home in an incident confirmed by Huthi television channel Al-Massira.
A journalist who won Amnesty International Special Award for Human Rights Journalism under Threat in 2009, he had been head of the militia's media department.
He represented the Huthis during national talks held under a UN-backed deal that ended a year of deadly protests against President Ali Abdullah Saleh, who stepped down in February 2012.
The Huthis have been effective masters of Sanaa since September, expanding their zone of control to neighbouring regions and to the shores of the Red Sea.
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President Abedrabbo Mansour Hadi escaped last month from house arrest in Sanaa and has fled to second city Aden in the south, where he is attempting to re-establish authority.
Tension remains high in Sanaa, where groups opposing the Huthis have been staging frequent demonstrations demanding their pullout and voicing support for Hadi.
A key US ally in the fight against Al-Qaeda, Yemen has descended into chaos since the departure of Saleh, who has been accused of backing the Huthis.