Mohammed Hila Hammad Obeidi – also known as Abu Ayman - was captured during an operation in southern Baghdad by US and Iraqi forces, the military said in a statement.
The statement said Ayman was wanted for a series of hostage killings, assassination attempts and roadside bombings, as well as the kidnapping of Italian journalist Guiliana Sgrena.
He was caught on March 7 but the announcement was delayed until DNA tests confirmed his identity, the military said.
According to the US, Ayman was an aide to the Chief of Staff of Intelligence in Saddam Hussein's government and has also led the Secret Islamic Army, a militia group operating in Babil province.
The Sgrena kidnapping attracted further attention after her release when US soldiers at a roadblock shot dead an Italian secret service agent moving her to safety.
Abu Qatada, an alleged lieutenant of Abu Ayman, was captured by the US military in December last year.