A judge confirmed the verdict against Hamid al-Ali on Sunday, according to his lawyer Usama al-Munawar.
"He rejected the appeal launched by the public prosecution which called for jailing my client," the lawyer added.
Al-Ali is the former secretary-general of the Salaf Movement and was originally sentenced on 19 June.
The appeals court supported another court order that al-Ali pay $3400 and maintain good conduct for two years after being accused of challenging the amir's constitutional rights, al-Munawar said.
It also backed a ruling that found al-Ali guilty of defaming Arab leaders by calling them "traitors" and "failures", and also of "insulting the [Kuwaiti] state's reputation".
Ali was additionally charged with calling for demonstrations and meetings without obtaining a licence.
The charges were based on Ali's fatwa, or religious edict, which stated that "any backing by a Muslim country of foreign troops in attacking another Muslim state contravenes Islamic teachings".
Amir Jabir al-Ahmad al-Sabah
(C) is a staunch ally of the US
The verdict is final unless the public prosecution appeals to the supreme court.
Several Islamic activists are still on trial on charges of carrying out attacks against US forces in Kuwait or for allegedly being associated with Usama bin Ladin's al-Qaida network.
Kuwait, which was invaded by Saddam's army in August 1990 and occupied for seven months, served as the main launchpad for last year's US-led war.
The invasion was overwhelmingly opposed by Muslim countries all over the world.