At least 120 prominent writers and artists from around the world have signed a letter sent to Egypt’s president urging him to release Ahmed Naji, an Egyptian author and journalist imprisoned for a novel deemed to have violated public morals. The letter, sent by PEN America, a group that promotes free expression, amplifies the international pressure on the Egyptian president, Abdel Fattah el-Sisi, over his increasingly harsh repression of writers and journalists. Mr. Sisi’s government has imprisoned an unprecedented number of journalists since he took power in 2013. His policy of intolerance toward views he dislikes recently aroused the ire of Egypt’s journalists union, which includes many who work for state-run news outlets. The PEN America letter was sent before the organization’s annual Literary Gala in New York on May 16, when Mr. Naji will be honored in absentia with the group’s annual Freedom to Write award. Mr. Naji, 30, has been a vocal critic of corruption under Mr. Sisi’s administration. He was charged last year with violating public morality provisions in the penal code for explicit references to sex and drug use in a novel, “The Use of Life,” and sentenced in February to two years in prison. In what clearly looked like a politically motivated prosecution, the courts ignored that Egyptian censors had approved the references. Moreover, critics of the morality provisions contend they violate protections of freedom of expression in Egypt’s 2014 Constitution.

“Writing is not a crime,” reads the letter to Mr. Sisi, which was signed by a diverse list of literary and artistic figures including Woody Allen, Margaret Atwood, J. M. Coetzee, Jessica Hagedorn, David Henry Hwang and Orhan Pamuk.

The letter called the sentencing “emblematic of the Egyptian government’s deeply troubling crackdown on free expression.”

The Egyptian authorities have not commented on the letter, which PEN America planned to announce on Monday.

Suzanne Nossel, the executive director of PEN America, said in a telephone interview that the organization had discussed the letter in advance with Mr. Naji’s lawyer and members of his family for advice on “what the best approach would be.”

The organization has sent such letters before to other governments, seeking pardons for imprisoned writers. But in this case, Ms. Nossel said, Mr. Naji did not want a pardon that would have let his conviction stand.

“He wanted an amendment to the law under which he had been convicted,” she said.

PEN America has what Ms. Nossel described as a good record in helping pressure governments to free imprisoned writers and artists who receive the organization’s annual award. Thirty-five of the 40 recipients who were incarcerated at the time they won were subsequently released.

“We can’t say that it’s direct causation, but we certainly believe the attention and pressure makes a difference, and many of the award winners have told us that,” Ms. Nossel said.

PEN America’s 2015 award winner, Khadija Ismayilova, an Azerbaijani investigative reporter who exposed government corruption, was sentenced to seven and a half years in prison on charges of abuse of power and tax evasion.

Although she is still imprisoned, the Azerbaijani authorities are feeling increased pressure over her case. On Tuesday, Ms. Ismayilova was awarded the annual World Press Freedom Prize by Unesco, the United Nations’ cultural heritage agency, in recognition of her “outstanding contribution to press freedom in difficult circumstances.”