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 a .380 handgun inside Matias' motorboat, they tried (and failed) to track him down for questioning.