The women's supporters in the Alexandria courtroom chanted "God is great" as the judge pronounced the ruling.
The girls, who had initially been sentenced to juvenile detention, were ordered freed by a separate court but placed on three months' probation.
The initial sentences in November had shocked even supporters of the military-installed government, with images of the white-clad defendants also galvanising the Islamist opposition.
The 21 were all convicted of taking part in a violent protest demanding Islamist president Mohamed Morsi's reinstatement following his overthrow by the army in July.
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The hearing by a juvenile appeals court for the seven girls was held in an adjacent courtroom.
Judge Sharif Hafiz found the 14 women guilty of three counts relating to violence during the protest, but reduced their sentence to one year and suspended it.
Their lawyer Ahmed al-Hamrawy had urged the court to acquit them, arguing there was no evidence against the women.
"Even in Mubarak's era there were morals. Egypt's women and girls were a red line and they weren't placed on trial," he told the court, referring to ousted dictator Hosni Mubarak, Morsi's predecessor.
She said her daughter was among the seven juveniles sentenced, and explained that they had both been near the October 31 protest by chance when arrested.
"I have the right to express my opinion -- this is a constitutional right, and we are currently political prisoners," said another defendant, Aya Adel.
The prosecutor general's office charged that the women fought with knives and threw rocks during clashes that erupted during the protest in Egypt's second city.
Six men said to be Muslim Brotherhood leaders were tried in absentia in the same case and sentenced to 15 years.
There was a heavy police presence outside the court complex in the coastal city, where Morsi's Islamist supporters have repeatedly clashed with opponents and security forces.
During the recess, about 100 friends and relatives of the defendants stood outside the courtroom chanting "Down with military rule."
The jail terms, coming in the same week as a restrictive new protest law, re-energised Islamist opposition to the interim government and drew criticism even from its secular supporters.
Hamdeen Sabbahi, a former presidential candidate and leading dissident under Morsi, called on the interim president to pardon the girls and repeal the new law governing protests.
Much of the Brotherhood's leadership, including Morsi, is on trial on various charges of inciting violence.
Courts have handed out harsh sentences to Islamist demonstrators, including 12 men imprisoned for 17 years after a violent protest in Cairo.