A poly(ethylene glycol)-based thermogel can capture an iron ion (Fe3+) through a crown ether-like coordination bond between the oxygen atom and metal ions, thus, providing a sustained Fe3+-releasing system. Poly(ethylene glycol)-l-poly(alanine) thermogel was used in this study. The polypeptide forms a rather robust gel, and the degradation products are a neutral amino acid, which provides cyto-compatible neutral pH environments during the cell culture. During the heat-induced sol-to-gel transition at 37 °C, tonsil-derived mesenchymal stem cells (TMSCs) and iron ions were incorporated, leading to the formation of a three-dimensional matrix toward neuronal differentiation of the incorporated TMSCs. The initial concentration of the iron ions was varied between 0, 15, 30, and 60 mM. About 10% of the loaded iron ions was released over 21 days, which continuously supplied iron ions to the cells. The incorporation of iron ions not only increased the gel modulus at 37 °C from 107 to 680 Pa, but also promoted cell aggregation with a significant secretion of the cell adhesion signal of FAK. Expression of biomarkers related to the neuronal differentiation of TMSCs, including NFM, MAP2, GFAP, NURR1, NSE, and TUBB3, increased 4-35-fold at the mRNA level in the Fe3+-containing system compared to that of the system without Fe3+. Immunofluorescence studies also confirmed pronounced cell aggregation and a significant increase in neuronal biomarkers at the protein level. This study suggests that an iron ion-releasing thermogelling system can be a promising injectable scaffold toward neuronal differentiation of stem cells.