The defendants had appealed the August 31 verdicts, claiming that confessed -- which they later recanted -- had been given under duress.
Supreme Court judge Abrashid Zhukenov told a news briefing in Astana today that all of the defendants were guilty.
Former security officer Rustam Ibragimov was sentenced to death, while his co-defendants received jail terms of between three and 20 years. Kazakhstan introduced a moratorium on executions in 2003.
The defendants included current and former security personnel and state officials, fueling opposition claims that the execution-style slayings were politically motivated rather than the result of a personal feud, as prosecutors alleged.
At his press conference, judge Zhukenov said claims of more senior official involvement were "unsubstantiated," a finding that is unlikely to appease critics of the investigation and ensuing trial.
The case has been followed closely by rights groups and the victims' families, who have dismissed the proceedings as a "farce" aimed at covering up the true organizers of the assassinations.
Sarsenbaev, a former government minister and ambassador to Russia, was among the most forceful antigovernment politicians in a country where violence or prosecution often claim key opposition leaders and authorities keep a tight lid on dissent.
The bodies of Sarsenbaev and two aides, Baurzhan Baibosyn and Vasily Zhuravliov, were found in a gorge outside Almaty on February 13.
Sarsenbaev was among the founders of the Naghyz Ak Zhol (True Bright Path) opposition party in April 2005, after a split in the Ak Zhol party.
Sarsenbaev had been the object of government pressure ahead of the presidential election in December 2005, when incumbent Nursultan Nazarbaev won in a landslide. The Organization for Security and Cooperation in Europe (OSCE) called that vote deeply flawed, citing ballot-stuffing, intimidation, interference, campaign restrictions, and other violations.
(Interfax-Kazakhstan, Kazakhstan Today)