In this study, an electrochemical sensor was developed for the determination of propham (PRO) in potato, human urine and river water samples based on the over-oxidized poly(thiophene) modified pencil graphite [PG/p(Thp)-Ox] electrode. Adsorptive stripping differential pulse voltammetry was used as a voltammetric method. The PG/p(Thp)-Ox electrode increased the oxidation peak current of PRO 30 times according to bare PG. Electrochemical polymerization and over-oxidation conditions were deeply optimized to increase the sensitivity of PG/p(Thp)-Ox. Characterization of PG/p(Thp)-Ox was performed via scanning electron microscopy and cyclic voltammetry analysis. Oxidation peak current value of PRO linearly increased with the concentration of PRO in the ranges of 0.005-1.0 mu M and 2.0-15.0 mu M. A very low limit of detection value (1.0 nM) was obtained. Intra-day and inter-day reproducibility of the PG/p(Thp)-Ox were determined as 2.96% (N: 10) and 4.77% (10 days), respectively. Satisfactory recovery values (98.07-104.4%) were observed with PG/p(Thp)-Ox electrode during the analysis of PRO in PRO-spiked potato, urine and river water samples. The results show that the PG/p(Thp)-Ox could be used safely in the determination of PRO.