62-year-old Xi is already General Secretary of the ruling Communist Party and Chairman of the Central Military Commission, which manages the People's Liberation Army (PLA).
Xi is now Commander in Chief of the military's Joint Operations Command Centre, state media reported, showing him visiting the centre wearing camouflage fatigues.
The Xinhua news agency and the state broadcaster CCTV both carried reports referring to Xi by the description for the first time after he visited the command centre on Wednesday.
Xi told the officers to closely follow the trends of global military revolution and strive to build a joint battle command system that meets the need of fighting and winning an informationised war.
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"All must be done with the ultimate goal of improving battle command capacities and measured by the standards of being able to fight and win wars," Xi said, urging a focus on solving conflicts and problems limiting joint battle command.
The command centre was set up as part of a major revamp of China's military structure, which also included the creation of a strategic rocket force to operate its missiles.
China's foreign policy has become increasingly assertive in recent years, especially on its claims to disputed territories in the resource-rich and strategic South China Sea.
However, analysts say his new title indicates he wants to be seen as a leader capable of commanding the military directly.
Experts say his appearance in military fatigues may also be a display of strength aimed at China's rivals.
China and several of its neighbours are locked in a territorial dispute over the South China Sea, which Beijing claims almost in its entirety.
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Meanwhile, the establishment of a joint battle command capacities improve military capabilities and the outcome of conflict, the PLA Daily, the mouthpiece of the Chinese military commented today.
The article urged military authorities to heighten their sense of crisis and adapt to the strategic demands of national security.
It said China's joint battle command system should support the needs of fighting and winning an informationised war and fulfil its missions and tasks efficiently.
The article also asked military personnel to strengthen their theoretical study, training and command disposal drills.
"If they did not grow to be experts of joint battle command, it would be difficult for the army to keep a firm grip on the future developments in conflicts," it warned.