The $^{135}$Cs/$^{137}$Cs isotopic ratio is a powerful tool for tracing the origin of radioactive contamination. Since the Fukushima accident, this ratio has been measured by mass spectrometry in several highly contaminated environmental matrices mainly collected near nuclear accident exclusion zones and former nuclear test areas. However, few data were reported at $^{137}$Cs environmental levels (<1 kBq kg$^{–1}$). This is explained by the occurrence of analytical challenges related to the very low radiocesium content at the environmental level with the large presence of mass interferences, making $^{135}$Cs and $^{137}$Cs measurements difficult. To overcome these difficulties, a highly selective procedure for Cs extraction/separation combined with an efficient mass spectrometry measurement must be applied on a quantity of ca. 100 g of soil. In the current research, an innovative inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) method has been developed for the $^{135}$Cs/$^{137}$Cs ratio measurement in low activity environmental samples. The use of ICP-MS/MS led to a powerful suppression of $^{135}$Cs and $^{137}$Cs interferences by introducing N$_2$O, He, and, for the first time, NH$_3$, into the collision-reaction cell. By adjusting the flow rates of these gases, the best compromise between a maximum signal in Cs and an effective interference elimination was achieved allowing a high Cs sensitivity of more than 1.10$^5$ cps/(ng g$^{–1}$) and low background levels at $m/z$ 135 and 137 lower than 0.6 cps. The accuracy of the developed method was successfully verified by analyzing two certified reference materials (IAEA-330 and IAEA-375) commonly used in the literature as validation samples and three sediment samples collected in the Niida River catchment (Japan) impacted by the Fukushima fallout.