P-gp is the most widely studied MDR protein conferring cellular resistance to many standard or targeted therapeutic agents. For this reason, P-gp chemoresistance evaluation, established before or during chemotherapy, can be very relevant in order to optimize the efficacy of treatments, particularly for aggressive tumoral subtypes such as triple-negative breast cancer (TNBC). In this context, our team developed an innovative cell-permeant fluorescent probe called the LightSpot®-FL-1, which is able to specifically localize and quantify the P-gp in cells or cell masses, as evidenced on different TNBC cell models. First, flow cytometry analysis showed LightSpot®-FL-1 cell penetration and persistence in time, in TNBC cells. Then, LightSpot®-FL-1 staining was compared to anti-P-gp immunostaining by fluorescence microscopy on five TNBC cell lines. Results showed a clear similarity of P-gp localization and expression level, confirmed by Pearson’s and Mander’s colocalization coefficients with 92.1% and 100.0%, and a strong correlation coefficient of R2 = 0.99. In addition, the LightSpot®-FL-1 staining allowed the quantification of a P-gp induction (33% expression increase) following a 6-hour spheroid model exposure to the anti-PARP Olaparib. Thus, the new LightSpot®-FL-1 cell-permeant probe, targeting P-gp, appears to be an effective tool for drug resistance evaluation in preclinical models and shows promising possibilities for future use in clinical diagnosis.