Phenol determination is an important step of its monitoring to prevent water contamination. Herein, a glassy carbon electrode (GCE) is modified with electrochemically reduced graphene oxide (ERGO) and horseradish peroxidase (HRP) enzyme (HRP/ERGO/GCE). Then, the modified electrode (HRP/ERGO/GCE) has been used for the biosensing of phenol. Using HRP, phenol is oxidized into electroactive o-quinone, which can be electrochemically reduced to catechol. The electrochemical responses obtained in the reduction of o-quinone to catechol are directly related to the phenol concentration. Also, the selectivity of phenol enzymatic oxidation is significantly increased when HRP is used to fabricate the biosensor. In order to enhance the electrochemical responses, ERGO is employed to fabricate the biosensor. The results indicate that the o-quinone/catechol redox couple is adsorbed on the HRP/ERGO/GCE surface and leads to the enhancement of the responses and the sensitivity of the biosensor to determine the phenol concentration. The voltammetric responses of the biosensor, during preparation under specified conditions, are also examined. Using differential pulse voltammetry, a wide linear range of 3.0–100.0 μM and a low detection limit of 2.19 μM are obtained for phenol determination at the HRP/ERGO/GCE surface. Finally, the proposed biosensor has been satisfactorily used to determine the phenol concentration in a tap water sample.