Injectable hydrogels are considered important to realize safe and effective minimally invasive therapy. Although animal‐derived natural polymers are well studied, they typically lack injectability and fail to eliminate the potential risks of immunogenic reactions or unknown pathogen contamination. Despite extensive research activities to explore ideal injectable hydrogels, such state‐of‐the‐art technology remains inaccessible to non‐specialists. In this article, the design of a new injectable hydrogel platform that can be extemporaneously prepared from commercially available animal‐component‐free materials is described. The hydrogels can be prepared simply by mixing mutually reactive aqueous solutions without necessitating specialized knowledge or equipment. Their solidification time can be adjusted by choosing proper buffer conditions from immediate to an extended period of time, that is, few or several tens of minutes depending on the concentration of polymeric components, which not only provides injectability, but enables 3D encapsulation of cells. Mesenchymal stromal/stem cells can be encapsulated and cultured in the hydrogels at least for 2 weeks by traditional cell culture techniques, and retrieved by collagenase digestion with cell viability of approximately 80%. This hydrogel platform accelerates future cell‐related research activities.