Electrical resistivity imaging surveys are widely conducted in waste disposal sites due to their ease of operation and accurate results; however, some essential measurement parameters, such as optimal electrode spacing, have not been thoroughly evaluated. Accordingly, this study aims to identify the optimal electrode spacing for electrical resistivity surveys in open dumpsites. An electrical resistivity survey was conducted at the Nonthaburi disposal site, Thailand, and the results were compared to synthetic cases simulated using the forward modeling technique. Electrode spacing values of 2, 2.5, 4, and 5 m were used. The models were evaluated using the Nash–Sutcliffe model efficiency coefficient, model sensitivity, and root mean square error. The real case survey results illustrate that small electrode spacings of 2 and 2.5 m provide a realistic model, however, the root mean square error is higher due to the use of more outlier electrodes during measurement. Consistent with the synthetic case studies, electrode spacing values of 2 and 2.5 m yielded accurate inverted models. Overall, this study illustrates the benefits of using a forward modeling technique for selecting the optimal electrode spacing for open dumpsite surveys.