Ms. Sotoudeh characterized her colleague as a law-abiding citizen who defended victims of acid attacks and voted while in prison during the 2015 parliamentary elections. “Every day Narges is behind bars is one too many,” Ms. Sotoudeh said.

Image Narges Mohammadi in Tehran in 2008. Credit... Vahid Salemi/Associated Press

Ms. Mohammadi’s lawyer, Massoud Behzadi, said: “I expect that she will only have to serve 10 out of 16 years. If Narges agrees we will appeal to the Supreme Court.”

The prison term comes as Iran’s government is trying to reach out to the West to restore political and business relations. In January, the country’s nuclear deal was put into effect and some of the economic sanctions against Iran were lifted.

Ms. Mohammadi’s original sentence was handed down by Abolghassem Salavati, a hard-line jurist who heads a revolutionary court that usually handles cases involving activists and Iranians with dual nationalities, a status Iran does not recognize.

He found her guilty of “gathering and conspiring with the aim of committing crimes against national security,” sentencing her to five years in prison. He added one year for “propaganda against the state” and 10 years for “forming and managing an illegal group.” Ms. Mohammadi ran Legham, an organization calling for the abolishment of the death penalty.