The transfer fee was not disclosed. The transaction opens a designated player spot and an international spot on United's 2020 roster.
Martinez, 27, tallied two goals and two assists in seven MLS matches this season. He added two goals and two assists in three games in Champions League competition.
In his first season with Atlanta in 2019, he was named an MLS All-Star with five goals and nine assists and helped the team win the U.S. Open Cup and the Campeones Cup.
"We want to thank Pity for his contributions to our club for the past two years," said Atlanta United vice president and technical director Carlos Bocanegra in a news release. "He will forever be a part of Atlanta United's history after playing an integral part in our 2019 season in which we lifted two trophies. We'll look to be diligent in our efforts to fill that roster spot during this window."
A native of Argentina, Martinez transferred from River Plate and joined Atlanta as a designated player on Jan. 24, 2019.