A key hurdle preventing translation of stem cell therapies to the clinic is the lack of efficient strategies to transplant and retain viable cells. Cell transplantation by direct injection is favored for its minimal invasiveness but commonly results in poor cell viability. Use of thixotropic hydrogels as injectable cell-delivery vehicles is one potential strategy to overcome this limitation. We report a protein-engineered hydrogel designed to meet four criteria for use in stem cell injection protocols: (i) gentle cell encapsulation at constant physiological conditions without the need for chemical crosslinkers, (ii) shear-thinning under reasonable hand-injection force through a syringe needle, (iii) rapid gel recovery to localize cells at the injection site, and (iv) cell-adhesive ligands and mechanical properties conducive to three-dimensional (3D) cell culture. This Mixing-Induced Two-Component Hydrogel (MITCH) is synthesized using protein-engineering technology to yield monodisperse block-copolymers that hetero-assemble upon simple mixing. Human and mouse adipose-derived stem cells (ASCs) remain viable and display a well-spread 3D morphology within MITCH. Use of MITCH to deliver ASCs to the subcutaneous dorsa of athymic mice resulted in significantly greater viable cell retention at the implant site compared to Type I collagen or buffer alone up to two weeks post-transplantation.