The older brother of soccer mega-star Lionel Messi is a wanted man in Argentina after cops discovered a gun in his blood-soaked boat ... according to reports.
Matias Messi allegedly told a security guard at the Fighiera fishing club that he cut his face by colliding with a sandbar ... but after police discovered the .380 handgun inside Matias' motorboat, they tried (and failed) to track him down for questioning.
The Messi family is pissed over all the accusations -- issuing a statement today saying Matias was NOT carrying a gun on the boat .