He was sentenced to 7 1/2 years in prison last month for his role in a mass strike by oil workers in the country's west that ended in violence in December 2011.
During the unrest, police shot dead 17 people in the western towns of Zhanaozen and Shetpe.
Kazakh human rights defender Yevgeny Zhovtis was present at the court hearing on November 19 and confirmed to RFE/RL that Kozlov's prison sentence was upheld.
Kozlov, 52, insists the case against him is politically motivated as his opposition group is critical of the government and authoritarian President Nursultan Nazarbaev.
Last week, Human Rights Watch urged Kazakhstan to hold an impartial appeal hearing for Kozlov, calling his sentence "a blow to freedom of expression and political pluralism."