Airport police discovered 58 grams of cannabis inside a can of ice tea, three pipes with cannabis oil, a THC candy and four glass containers of compressed marijuana weighing 4.3 grams, officials told The Costa Rica Star.
Harmon arrived from Fort Lauderdale, Florida, was briefly detained and then sent back to the United States.
"This involved a professional NFL player, who tried to come into the country carrying marijuana," Irving Malespín, director of the Fiscal Control Police said. "He has sent back to US territory. It is important to highlight the work of different police bodies, who carry out important operations in different areas of the country. We want to prevent undesirable people from entering the country."
"We are aware of the situation involving Duron Harmon Friday night in Costa Rica," the Patriots said in a statement. "He has since returned to the U.S. and we are seeking to gain more information. At this time, we have no further update."
Harmon posted an apology to the Patriots and his family on social media, saying "I apologize with all my heart."
"Friday does not represent who I am or the person I strive to be," Harmon said. "I am committed to doing everything within my power to regain the trust and respect of everyone who believes in me as an athlete, role model, person and friend."
The 27-year-old Harmon had four interceptions and 23 tackles last season.
He has been with the Patriots for his entire five-year NFL career.