General Li Shangfu, who has been absent from public view for two months, was dismissed as defence minister and state councillor, according to state media.
China also announced that Qin Gang, who was removed as foreign minister in July, was also stripped of his state councillor position.
China's top legislators, the Standing Committee of the National People's Congress, approved the removal of both men, state broadcaster CCTV reported, without given any explanation.
No replacement for Li was named, leaving the country without a defence minister as it prepares to host foreign defence officials at the Beijing Xiangshan Forum on October 29-31.
Li, 65, disappeared from public view two months ago.
Reuters reported last month that he was under investigation for suspected corruption related to equipment procurement and development.
Li was placed under US sanctions in 2018 over China's purchase of combat aircraft and equipment from Russia.
He had only been in the job since March, when Xi started his precedent-breaking third term as head of state.
No other defence minister in China had served for a shorter time than Li.
Qin had also served less than a year before he disappeared from public view and was replaced by his predecessor, Wang Yi.
No formal explanation has been given but the Wall Street Journal cited sources saying Qin had an extramarital affair while he was ambassador to the United States.
With the ousting of both men, the number of China's state councillors - a more senior rank than minister - is down to three.
