The isotopic signature of radionuclides provides a powerful tool for discriminating radioactive contamination sources and estimating their respective contributions in the environment. In this context, the $^{135}$Cs/$^{137}$Cs ratio has been tested as a very promising isotopic ratio that had not been explored yet in many countries around the world including France. To quantify the levels of radioactivity found in the environment, a new method combining a thorough radiochemical treatment of the sample and an efficient measurement by ICP-MS/MS has been recently developed. This method was successfully applied, for the first time, to soil and sediment samples collected in France in two mountainous regions preferentially impacted either by global fallout from nuclear weapons testing (i.e., the Pyrenees) or by the Chernobyl accident (i.e., the Southern Alps). The $^{135}$Cs/$^{137}$Cs ratios measured on twenty-one samples ranged from 0.66 ± 0.04 and 4.29 ± 0.21 (decaycorrected to January 1 st , 2022) corresponding to the characteristic signatures of the fallout from Chernobyl and global fallout associated with the nuclear weapons testing, respectively. Moreover, large variations of both the $^{137}$Cs mass activity and the studied isotopic ratio recorded by most samples from the southern Alps suggest varying proportions of these two $^{137}$Cs sources. For these samples, the contribution of each source was estimated using this new tracer ($^{135}$Cs/$^{137}$Cs) and compared with the mixing contribution given by activity ratio: $^{239+240}$Pu/$^{137}$Cs. This work has successfully demonstrated the applicability of the $^{135}$Cs/$^{137}$Cs isotopic signature to nuclear forensic studies and could be extended to better evaluate the environmental impact of nuclear facilities (i.e., NPP, waste reprocessing).