The trial was held at a sports stadium in Xinjiang Province, in western China, where about 7,000 spectators watched the trial.
The accused appeared in orange jumpsuits and were positioned on the back of pick-up trucks, surrounded by security forces.
A banner at the stadium read: "Yili Prefecture Public Judgment, Public Seizure, Public Arrest General Assembly."
Charges against the 55 accused included homicide, separatism, and terrorist activities. Three people were sentenced to death for murdering a family of four "with extreme cruelty."
"We have the confidence, resolution and capability to take a firm stance, form an iron hand and hit hard and fast like a thunder to vanquish the enemy's arrogant audacity," Li Minghui, vice-secretary of the prefectural Communist Party committee said. "We will achieve the final victory of this war of the people to stop terrorism and maintain stability."
Human rights organizations criticized the mass trial. Amnesty International's William Nee, a China researcher, said that while "Those responsible for the recent violent attacks have show a callous disregard for human life and must be held to account ... speedy show trials will not deliver justice for the victims."
There have been terror attacks across China recently that have been linked to Xinjiang's Uighur Muslim minority population.
"Hastily sentencing people after unfair trials will only exacerbate tensions," Nee cautioned.
The mass terror trial follows China's "one-year crackdown on violent terrorist activities," announced earlier this month.