For the first time, a simple and sensitive electrochemical sensor based on a screen printed electrode (SPE) modified with titanium dioxide (TiO2) and polytriazine imide submicrostructured composite (TiO2-PTI) has been developed for the simultaneous detection of fipronil (FIP) and its toxic metabolite fipronil sulfone (FIP-S). The submicrostructured composite material based on TiO2 and PTI was obtained by simple hydrothermal treatment of the Ti peroxocomplexes in the presence of pristine. This carbon nitride allotrope has better crystallinity and conductivity than its graphitic analog. It was found that the TiO2-PTI submicrostructured composite enhanced the electrochemical sensing of the SPE electrode towards FIP and its metabolite FIP-S in 0.1 M Britton-Robinson buffer (pH 10) at the oxidation potentials of 0.82 V and 0.94 V, respectively. In addition, it showed good stability and reproducibility for the determination of both analytes. Under optimal conditions, the peak currents by square wave voltammetry were found to vary linearly with FIP and FIP-S concentrations in the range from 0.01 to 10 μM and from 10 to 50 μM, with a detection limit of 8.42 nM, 3.6 μg/kg for FIP and 9.72 nM, 4.04 μg/kg for FIP-S. This sensor was successfully used to detect FIP and FIP-S in eggs and water samples with good recoveries of 90%–106.6%, which was validated by liquid chromatography tandem mass spectrometry.