Giardina has been demoted to a two-star admiral and re-assigned to a Navy staff position pending the outcome of the gambling investigation, Navy spokesman Read Adm. John Kirby told the Associated Press, which first reported Giardina's firing.
Giardina was named the No. 2 official at Strategic Command, which overseas the U.S. nuclear arsenal, in December 2011.
He is a career submarine officer who was previously deputy commander and chief of staff at the U.S. Pacific Fleet command in Honolulu.
— This story was updated at 6:25 p.m.