Dozens of people were also wounded in the attack in the mainly Sunni city of Ibb, held by the Huthi Shiite militia, the medics said, warning that the death toll was likely to rise.
Yemen has been dogged by instability since an uprising forced longtime strongman Ali Abdullah Saleh from power in 2012, with Huthi rebels and Al-Qaeda seeking to fill the power vacuum.
Among those wounded in Wednesday's attack was Ibb governor Yahya al-Aryani, who was attending the Huthi commemoration of the Prophet Mohammed's birthday, a medic said.
A security source told AFP the bomber had been wearing a woman's black abaya and veil when he detonated his suicide belt.
Another bomb was discovered at the entrance to the event's venue, the source added.
The United States, allied with Yemen in fighting militants, "strongly condemns" the attack, a State Department spokesman said.
"We will continue to stand with the government of Yemen and the Yemeni people to counter the shared threat posed by violent extremists," added Jeff Rathke, the spokesman.
Al-Qaeda in the Arabian Peninsula, the Yemen branch of the jihadist organisation, has pledged to fight the Huthis, who overran the capital Sanaa unopposed in September and have since advanced into mainly Sunni districts such as Ibb.
Signup to our newsletter and follow us on Facebook and Twitter!
AQAP military chief Qassem al-Rimi vowed in November to carry out fierce attacks against the Shiite militia, and several deadly bombings targeting Huthis have been claimed by the jihadist group.
Al-Qaeda has also carried out a spate of attacks against the security forces from its strongholds in the south and east in recent weeks.
In the southern province of Shabwa on Wednesday, unidentified gunmen killed police officer Colonel Mohamed al-Nessi near the provincial capital Ataq before fleeing, a police source said.
There was no immediate claim for the shooting but last weekend AQAP claimed an attempt to assassinate a top general in an ambush that killed two soldiers and wounded 11.
In a second incident, AQAP said its gunmen opened fire on an army colonel in central Baida city, killing him instantly.
And on Tuesday unidentified gunmen on a motorcycle shot and killed an army officer in the southern port city of Aden.
AQAP is considered by the United States to be the global jihadist network's most dangerous branch and its attacks on security forces challenge Yemen's longtime role as an ally in Washington's fight against extremists.
Its proximity to oil-rich Saudi Arabia and key shipping routes in the Gulf of Aden have also raised fears that it could become a failed state similar to Somalia.
The US slapped sanctions on former president Saleh in November for allegedly supporting the Huthi advance.
It also blacklisted two Shiite rebel commanders, Abdullah Yahya al-Hakim and Abd al-Khaliq al-Huthi, "for engaging in acts that directly or indirectly threaten the peace, security, or stability of Yemen."