Engineering extracellular matrices that utilize the body's natural healing capacity enable the progression of regenerative therapies. Fibrin, widely used as a surgical sealant, is one such matrix that may be augmented by the addition of protein factors to promote cell infiltration and differentiation. The thrombin-catalyzed conversion of fibrinogen to fibrin exposes N-terminal fibrin knobs that bind to C-terminal pockets to form the fibrin network. Here, we have created a platform system for the production of therapeutic proteins that capitalize on these native knob:pocket interactions for protein delivery within fibrin matrices. This system enables the retention of therapeutic proteins within fibrin without additional enzymatic or synthetic crosslinking factors. Using an integrin-binding fibronectin fragment as a model protein, we demonstrate that engineered knob–protein fusions bind consistently and specifically to fibrin(ogen). Equilibrium dissociation constants (KD) obtained using surface plasmon resonance indicate that these fusions have µm binding affinities, comparable to the native knob-containing fibrin fragments. The specificity of these interactions was verified by ELISA in the presence of molar excess of competing knob mimics. Release profiles and real-time confocal imaging demonstrate that the fusions were retained within fibrin matrices, even under the stringent continuous perfusion conditions used in the latter. In summary, this work explores the benefits and limitations of engaging native, biologically-inspired, non-covalent knob:pocket interactions within fibrin(ogen) for the retention of therapeutic proteins in fibrin matrices and provides insight into the stability of native knob:pocket interactions within fibrin networks.