Background: The remote dielectric sensing (ReDS) system is a recently introduced non-invasive technology used to easily estimate the degree of lung fluid volume without any expert techniques. In the previous literature, ReDS values had a moderate correlation with invasively measured pulmonary artery wedge pressure (PAWP), the gold standard for representing left heart preload. Considering the mechanism of ReDS technology, ReDS values may be inappropriately elevated in the presence of pleural effusion (PE), and the ability of the ReDS system to estimate PAWP may decrease in such a situation. Methods: In-hospital patients with cardiovascular diseases underwent computed tomography, and the presence of pleural effusion (PE) was evaluated. The measurement of ReDS values using the ReDS system and the measurement of PAWP using invasive right heart catheterization were also performed simultaneously. The impact of the presence of PE on the relationship between the ReDS values and PAWP was evaluated. Results: A total of 59 patients (79 years, 30 male) were included. The median ReDS value was 25% (IQR: 22%, 30%) and the median PAWP level was 13 (IQR: 10, 18) mmHg. Of these patients, 11 had PE. PAWP was not significantly different between the two groups, whereas the ReDS values in the PE group were significantly higher than in the no-PE group. The impact of the presence of PE on the ReDS values was significant, with a beta value of 6.61 (95% confidence interval: 4.80–8.42,p< 0.001) upon adjusting for the levels of PAWP. Conclusions: We should pay attention to interpreting ReDS values when assessing the degree of pulmonary congestion in patients with PE, because ReDS values may be inappropriately elevated in this cohort.