Tumor Treating Fields (TTFields) is an FDA-approved technique used in the treatment of glioblastoma (GBM). It consists in inducing an electric field (EF) in the head in two perpendicular directions alternately using a specific device that operates at 200 kHz. Post-hoc analyses of the EF-14 clinical trial results showed that an intensity of at least 1.06 V/cm and a minimum power density (PD) of 1.15 mW/cm3 at the tumor bed can significantly affect the mitotic process of tumoral cells. Given that it is not possible to experimentally measure these quantities, computational modelling is widely used to predict treatment effectiveness. However, the uncertainties in the electric conductivity (σ) of biological tissues affect the accuracy of these predictions. The goal of this work is to investigate for which tissues the uncertainty associated with σ values leads to the highest variations in these metrics and quantify their impact. A realistic head model divided into scalp, skull, CSF and brain was used in this study. A virtual lesion was added at the right hemisphere near the ventricle. This lesion intended to represent a GBM tumor and it consisted in a necrotic region surrounded by an enhancing part. Two pairs of transducer arrays were placed on the scalp. The first was placed over the supraorbital region and at the back of the head (Anterior-Posterior or AP configuration) and the other over the left and right temporal and parietal regions (Left-Right or LR configuration). To perform the sensitivity analysis of the most significant electric parameters, an extensive literature review was performed to obtain a range of σ values for each tissue represented. The average values of the EF intensity and PD were analyzed in a volume of interest (VOI) containing the enhancing tumor plus a 3-mm-thick proximal boundary surrounding it. Given that this array layout was not optimized for this specific tumor location the AP configuration did not produce a therapeutic EF at the tumor bed and thus just the results for the LR configuration were analyzed. The highest variation in the EF intensity in the VOI was seen when changing the electric conductivity of the white matter. Decreasing it led to a maximum of 1.82 V/cm, whereas increasing it to the maximum value reported in the literature led to a minimum EF intensity of 1.14 V/cm. On the other hand, the minimum value for the PD occurred when skull’s conductivity was reduced, 1.16 mW/cm3, while the maximum was obtained when scalp’s conductivity was diminished, 2.69 mW/cm3. These results indicate that the uncertainty associated with the electric parameters of tissues might lead to significant variations in the metrics used to evaluate TTFields efficacy. Experimental measurements are necessary to determine these values with precision and thus to increase the accuracy of data obtained through computational simulations.