The deal was reached via negotiations with prosecutors, according to Argentinian outlet La Capital.
The firearm was reportedly retrieved from a blood-stained speedboat at the end of last year when Matias, 35, crashed on the Paraná River, the second longest stretch of water in South America. His legal team deny that he was the owner of the weapon.
Matias suffered a broken jaw and other injuries in the incident which happened, according to his lawyers, when he crashed into a sandbank.
The Barcelona star’s brother has been in hot water for firearm offences in the past. He was held by police in 2015 after an unlicensed gun was discovered in his car. In 2008, he was arrested for allegedly carrying a loaded gun in his belt.
Matias is the second oldest of the Messi brothers, behind Rodrigo, 38, and ahead of Lionel who is 31. They have a sister, Maria, who is also 31.