A new type of pH-responsive indole-based (4-HINF) hydrogel, fabricated by a sol–gel method, was utilized as a platform for colorimetric detection of urea in aqueous solution. The colorimetric sensor was established by virtue of the synergistic effect of cation–π interaction and hydrogen bonding with good regenerative ability. The results exhibited linear response in the range of 0–10 mM with a limit of detection of 10 μM. The prepared 4-HINF hydrogel possessed high selectivity to pH change under complicated environments ensuring further applications in environmental and bio-systems.