The State Department said it would offer the reward for information leading to the arrest and conviction in any country of Maalim Ayman, leader of the Al-Shabab unit Jaysh Ayman.
"Ayman was responsible for preparing the January 2020 attack," a State Department statement said.
Al-Shabab, the Somali movement designated a terrorist group by Washington since 2008, claimed responsibility for the attack at the Manda Bay Airfield on Kenya's northern coast.
Authorities say the Jaysh Ayman unit carried out the pre-dawn raid, which killed two U.S. military service members and a US defense contractor and destroyed several aircraft.
The United States has worked closely both with Kenya and the fragile government in Mogadishu to counter Al-Shabaab, which has lost ground inside Somalia in recent months under pressure from an African Union force and US air strikes.
A study last year by George Washington University's Program on Extremism said that Jaysh Ayman was formed by Al-Shabab in an effort to penetrate into Kenya.
The unit, which has exercised growing autonomy, includes foreigners, dual nationals and Kenyans both of Somali and non-Somali descent, it said.