Al Samad, the president of the Houthi administration that runs most of northern Yemen, was killed in a Saudi-led coalition air strike in the Red Sea port city of Hodeidah in April that year.
He was the most senior official to be killed by the coalition in the years-long conflict. The Houthis are fighting forces loyal to Yemen's internationally recognised government.
Yemen's government is backed by a Saudi-led coalition that has received support from western powers. Saudi Arabia and Gulf Arab states point to the Houthis' Iranian backing as evidence that they are a proxy for Tehran.
The Houthi authorities said the nine men were charged and convicted for their involvement in Al Samad's killing, as well as for spying and sharing sensitive information with the coalition.
The rebels said the execution by firing squad was witnessed by "masses", including blood relatives and Houthi leaders in Sanaa, which they control.
The Yemeni capital was seized in a 2015 coup, starting a war that has left thousands dead and the country in the grip of the world's worst humanitarian disaster.