Critics claimed the case was built on fabricated evidence.
"This is a sad decision. It is sad not only for the families of the officers and the defendants, but for the whole of Turkey," defense lawyer Celal Ulgen told private NTV television. "Such a decision was handed down despite illegal evidence."
Ulgen said the high court's decision on the 237 defendants would now be appealed at either Turkey's supreme court or at the European Court of Human Rights.
There was no immediate comment on the appeals court decision from Prime Minister Recep Tayyip Erdogan's government. The government has in the past hailed the trial, which began in December 2010, as a move toward greater democracy.
Defendants whose convictions and sentences were confirmed include former air force chief Ibrahim Firtina, former navy chief Ozden Ornek, former army commander Cetin Dogan, and Engin Alan, a former general elected to Parliament two years ago. The top officials were accused of being among the masterminds of the plot.
The trial of the high-ranking officers inconceivable in Turkey a decade ago has helped significantly to tip the balance of power in the country in favor of civilian authorities.
Turkey's generals have staged three coups since the 1960s and forced an Islamist government to quit in 1997.
But Erdogan's government has grown more confident with each of its three electoral successes since 2002, and has been limiting the powers of the armed forces.