The verdicts were handed down on Monday by a court in the regional capital Urumqi, the scene of the violence that pitted mainly Muslim Uighurs against China's Han ethnic majority, leaving nearly 200 dead and over 1,600 injured.
"They were sentenced yesterday by the Urumqi Intermediate Court," a spokesman at the Xinjiang government who gave only his surname, Li, told AFP news agency.
"Four people were sentenced to death, one was sentenced to death with a two-year reprieve and eight others got prison terms of up to life."
A death sentence with a reprieve is normally commuted to life in prison.
According to the Xinjiang Daily newspaper the verdicts came in five separate cases with the 13 defendants charged with the "violent crimes of attacking, smashing, looting and burning", a Chinese term that refers to violent rioting.
|Ethnic Uighurs in western Xinjiang region have long complained of Chinese repression [EPA]
The defendants' names provided by the newspaper appeared to be Uighur, the Turkic-speaking Muslim minority group that has long complained of Chinese repression.
During the initial eruption of violence on July 5, Uighurs attacked Han Chinese, but in subsequent days groups of Han roamed the streets seeking revenge.
Uighurs say the violence was sparked when police cracked down on peaceful demonstrations in Urumqi that were held to protest the deaths of two Uighur migrant workers at a factory in southern China.
Authorities quickly implemented a clampdown on communications in and out of Xinjiang after the riots, blocking internet access, text messages and international phone calls.
These restrictions have only just been lifted, although the internet is still only partly accessible.
Earlier this month, state media said that funding for public security in Xinjiang would nearly double in 2010.
A budget proposal placed before Xinjiang's legislature called for $423 million to be spent on public security, up from $225 million in 2009, the official China Daily reported.
But exiled Uighurs say Beijing exaggerates the alleged separatist threat to justify harsh controls in the strategic western region, which is rich in energy reserves and borders on several central Asian countries.