Hydrogels containing redox-sensitive colorimetric nanoparticles (NPs) have been used to sense ambient pH in many fields owing to their simple and fast visualization capabilities. However, real-time pH monitoring still has limitations due to its poor response rate and irreversibility. Herein, we developed a fast responsive colorimetric hydrogel called ferrocene adsorption colorimetric hydrogel (FACH). Ferrocene, an organometallic compound, plays a vital role as an electron transfer mediator (i.e., redox catalyst) within the hydrogel network. FACH shows fast color change performance with high reactivity and penetrability to ambient pH changes. In detail, FACH shows distinct color change within 2 min under various pH conditions from four to eight, with good reliability. The speed for color change of FACH is approximately six times faster than that of previously developed colorimetric hydrogels, suggesting the fastest hydrogel-based colorimetric pH sensor. Furthermore, FACH shows reversibility and repeatability of the redox process, indicating scalable utility as a sustainable pH monitoring platform.