Following her arrest, China also detained former Canadian diplomat Michael Kovrig.
Schellenberg, who was reportedly detained in northeast Liaoning province in 2014, is accused of playing an important role in drug smuggling and of potential involvement in worldwide organised crime.
The man was sentenced to 15 years in prison in 2016, but filed an appeal.
Schellenberg was told in court he has the right to appeal within 10 days of receiving the ruling. The apparent weak evidence against the Canadian, the speed in scheduling a retrial, and China's invitation of the worldwide press to cover the sentencing all indicate that the move could be retaliatory and aimed at gaining leverage over Ottawa.
The Dalian court said Schellenberg, who was detained in December 2014, played a "key part" in an worldwide drug trafficking syndicate.
"It is obvious ... that Schellenberg's fate will have little to do with his actual guilt or innocence", Donald Clarke, a professor at George Washington University specializing in Chinese law, told the South China Morning Post.
"If the Chinese government has an innocent explanation for all the unusual features of this case, I hope it will provide it".
Schellenberg, 36, has been detained in China's Liaoning province since his arrest in 2014 for allegedly plotting to smuggle almost 500 pounds of crystal methamphetamine out of the country to Australia. The retrial on Monday was attended by about 70 observers including a small group of foreign journalists.
Huawei Chief Financial Officer Meng Wanzhou was arrested at the Vancouver airport on December 1 at the request of the United States, which wants her extradited to face fraud charges.
Beijing has repeatedly denied any diplomatic pressure behind Schellenberg's case.
The Chinese foreign ministry said Friday critics "can stop recklessly suspecting others of politicising legal issues just because they have done so".
Lu Kang, China's Foreign Ministry spokesperson.
"I am a normal person", he said in court Monday.
China has executed other foreigners for drug-related crimes in the past, including a Japanese national in 2014 and a Filipino in 2013. "Schellenberg could, for example, be sentenced to death with a two-year suspension".
But prosecutors said they had evidence that "highly suggests Schellenberg was involved in organised global drug crime", it reported quoting Chinese state-run broadcaster CGTN.
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