Mohammed al-Jishy told AFP the verdict was announced in the absence of Khawaja, who said on Sunday she would not attend the hearing because it was "impossible to have a fair and independent trial" in the Gulf kingdom.
Khawaja, a director of the Beirut-based Gulf Centre for Human Rights, also holds Danish nationality and is a daughter of jailed top Shiite opposition figure Abdulhadi al-Khawaja.
She has been in London since a travel ban was lifted on October 1, Jishy said.
Khawaja was arrested after arrival at Manama airport on August 30 on charges of hitting two policewomen. She insists it was police who attacked her.
She was granted bail on September 18 pending trial.
Her father was jailed for life following 2011 Shiite-led protests that were crushed by authorities in the Sunni-ruled state.
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She has been active abroad in criticising the Bahraini authorities since the crackdown.
"Today's sentence... is a powerful warning to human rights activists who criticise the regime," said Bryan Dooley, head of Human Rights Defenders Programme at the US-based Human Rights First.
"It tells those based out of Bahrain they risk jail if they try to return," he added in a statement.
Amnesty International urged the authorities to overturn the jail sentence.
"Maryam Al-Khawaja appears to be targeted for exposing the human rights violations taking place in Bahrain since 2011," said Amnesty's Middle East and North Africa deputy director Said Boumedouha.
"The authorities must ensure her conviction and sentence are quashed and drop any pending charges against her," he added in a statement.
Tiny but strategic Bahrain, home base of the US Navy's Fifth Fleet, remains deeply divided three years after the month-long protests.