The present invention relates to an improved osteosynthesis modular prosthesis, particularly for humerus osteosynthesis, characterize in that it comprises a cup element (7) having a convex surface suitable to be introduced within a bone cavity, particularly within the humerus glenoid cavity, and having a concave surface suitable to contain and keep joined all the bone fragments, particularly fragments of humerus epiphysial pa (2, 3, 4), permitting vascularization and the consequent recovery of bone tissue, a stem element suitable to be introduced within a diaphyseal channel, particularly within humerus diaphyseal channel, a coupling element (9), and coupling means (10, 11) suitable to fix ends of said coupling element (9) respectively at said cup element (7) and at said stem element (8).