Silanization of biomacromolecules has emerged as a fruitful approach to prepare hybrid biohydrogels. However, very little is known about interactions between organosilanes and biopolymers in solution. Here we focused on fibrin, a protein of interest in the biomedical field, whose self-assembly process and resulting gel structure are highly sensitive to experimental conditions. Three main silanes were selected to decipher the relative influence of the silanol groups and organic functions. Whereas no protein denaturation was observed, silanes bearing hydrophobic groups had a surfactant-like behavior and could improve the dispersion of fibrinogen molecules, impacting gel formation kinetics and rheological properties. 3D cultures of myoblasts evidenced that organosilanes could promote or impede cell proliferation, suggesting interactions of silanols with fibrin. These results demonstrate that the two sides of the coin of organosilane reactivity are relevant at different stages of fibrin gel formation and must be considered for future development of hybrid biomaterials.