Natural extracellular matrix (ECM) consists of complex signals interacting with each other to organize cellular behavior and responses. This sophisticated microenvironment can be mimicked by advanced materials presenting essential biochemical and physical properties in a synergistic manner. In this work, we developed a facile fabrication method for a novel nanofibrous self-assembled peptide amphiphile (PA) and poly(ethylene glycol) (PEG) composite hydrogel system with independently tunable biochemical, mechanical, and physical cues without any chemical modification of polymer backbone or additional polymer processing techniques to create synthetic ECM analogues. This approach allows noninteracting modification of multiple niche properties (e.g., bioactive ligands, stiffness, porosity), since no covalent conjugation method was used to modify PEG monomers for incorporation of bioactivity and porosity. Combining the self-assembled PA nanofibers with a chemically cross-linked polymer network simply by facile mixing followed by photopolymerization resulted in the formation of porous bioactive hydrogel systems. The resulting porous network can be functionalized with desired bioactive signaling epitopes by simply altering the amino acid sequence of the self-assembling PA molecule. In addition, the mechanical properties of the composite system can be precisely controlled by changing the PEG concentration. Therefore, nanofibrous self-assembled PA/PEG composite hydrogels reported in this work can provide new opportunities as versatile synthetic mimics of ECM with independently tunable biological and mechanical properties for tissue engineering and regenerative medicine applications. In addition, such systems could provide useful tools for investigation of how complex niche cues influence cellular behavior and tissue formation both in two-dimensional and three-dimensional platforms.