A British tourist who was imprisoned for stealing ancient artifacts from an archaeological site has had his conviction overturned by an Iraqi court.

The tourist, James Fitton, a 66-year-old retired geologist, was initially sentenced to 15 years in prison and had been detained for more than four months. Looting antiquities with weapons or with other people is a crime that is punishable by death in Iraq.

Mr. Fitton was arrested in late March after airport security confiscated 22 pieces of pottery and stones found in his luggage as he was leaving the country.

He confessed in court that he had picked up the artifacts while taking a tour of the Sumerian site of Eridu in the south, saying he did not know the act was punishable by law. Iraq’s Criminal Court found him guilty in June of attempting to smuggle the items abroad.

“We are celebrating a great victory — my client is free,” Thair Soud, Mr. Fitton’s defense lawyer, said in an interview. He expected his client would be released over the next few days from the Tasfirat prison in Baghdad, where he has been held since Jun. 6. “We are all relieved. It is a victory for my client and for the Iraqi judicial system,” he said.

According to a decision by Iraq’s Federal Cassation Court on Tuesday, provided by Mr. Soud, his client’s original sentence mistakenly relied on “insufficient evidence” and Mr. Fitton would be released from prison “immediately unless there is an impediment.”

Iraq’s Cassation Court said in its decision that Mr. Fitton, who “entered Iraq as a tourist and legally,” had picked up objects that “were located in vast and scattered places on the ground, unguarded, and their appearances seemed to suggest they are ordinary stones, not antiques.”

Mr. Soud, who had expressed surprise at the initial sentencing, said that he defended his client’s case with a four-page memo outlining errors by the Criminal Court.

Mr. Fitton had also wrapped the objects in Kleenex in his luggage, indicating that “these were souvenirs, not treasures someone was trying to hide,” he said.

Another German tourist who was traveling in Mr. Fitton’s group, Volker Waldmann, who had also been found with two artifacts in his luggage, was acquitted in June after he said he had been carrying the objects for Mr. Fitton.

The severity of Mr. Fitton’s sentence attracted immediate global attention in a country where looting is frequent but crimes against Iraqis are rarely met with such harsh punishment. Mr. Soud was met with an outpouring of support for his client on social media, he said, from Mr. Fitton’s friends and family as well as Iraqis.

More than 350,000 people signed an online petition organized by Mr. Fitton’s family to call for his release.

“We were informed this morning that the appeals court has decided to quash the verdict of the Felony Court,” Mr. Fitton’s son-in-law, Sam Tasker, wrote in a post on the petition’s webpage on Wednesday. “Once he is home, we will celebrate and take some time to recover as a family, and will be happy to tell the story to anyone who will listen.”

“The strain on his family has been very difficult,” said Mr. Soud, adding that he had remained in touch with them daily. Mr. Fitton, his lawyer said, had remained in good spirits.

“Throughout his detention my client has remained calm and hopeful,” Mr. Soud said, adding that Mr. Fitton’s living conditions in the prison were good. His client had never once complained, he said.

Mr. Fitton “is a good man and will even be missed,” Mr. Soud said, adding that his client had managed to earn the respect of prison authorities and inmates during his time in Baghdad. “But everyone would like him to return home to his family.”

Falih Hassan and Sangar Khaleel contributed reporting.