A foam consisting of reduced graphene oxide was synthesized by a one-pot hydrothermal method. The foam was used to prepare a nanocomposite with hemin which is formed via π-interactions. The nanocomposite was incorporated via a Nafion film and then placed on a glassy carbon electrode (GCE). The modified GCE displays outstanding catalytic activity towards H2O2. It is assumed that this is due to (a) the redox-active center [Fe(III/II)] of hemin, and (b) the crosslinked macroporous structure of the foam. Both improve the electron transfer rate and electrochemical signals. Under the optimum experimental conditions and a working voltage of typically −0.41 mV (vs. SCE), the sensor has a 2.8 nM H2O2 detection limit, and the analytically useful range extends from 5 nM to 5 mM with a sensitivity of 50.5 μA μM−1 cm−2. The modified GCE has high sensitivity and fast response. It was utilized to quantify H2O2 in spiked environmental water samples. Graphical abstract Schematic representation of the electrochemical sensor based on a nanocomposite prepared from hemin and reduced graphene oxide foam, which can be applied to the determination of hydrogen peroxide in serum.