Secretary general of the Union of Popular Forces party Mohammed Abdulmalik al-Mutawakil was shot dead by a gunman on a motorbike on a street in central Sanaa, his daughter Radhia told AFP.
Mutawakil was "fatally shot in the neck," by his assailants who fled, she said.
A medical source confirmed the death.
It was not immediately clear why Mutawakil was targeted.
The attack came a day after Shiite rebels and their political rivals, including Mutawakil's party, signed an agreement on the formation of a new technocrat government to resolve the country's crisis.
More From This Section
The overnight attack targeted security headquarters in Hudeida, which Shiite Huthi rebels seized in mid-October as they expanded across several towns and cities with almost no resistance from government forces.
Al-Qaeda frequently carries out attacks on Yemen's security forces and it has been locked in deadly battles with the Huthi rebels.
Dozens of Al-Qaeda suspects took part in the attack on the security forces headquarters in Jabal Ras, in Hudeida, "killings 13 soldiers, wounding 10, and capturing 15 others," a security official said.
Army reinforcements were sent to Jabal Ras and negotiations launched with the assailants to withdraw from the building, a tribal dignitary said.
"Tribal mediation is underway to secure the withdrawal of Al-Qaeda militants and the release of the captured soldiers," the source told AFP.
Yemen is a key US ally that has allowed Washington to conduct drone strikes against Al-Qaeda on its territory.
The Huthis easily overran Sanaa in September before moving on to Hudeida as well as Shiite-populated Dhamar and neighbouring Ibb.