Fast inhibition in the brain is mostly supported by chloride currents through the GABAA receptors. The driving force of these currents depends on the intracellular concentration of chloride ([Cl-]i) that is an important, but little studied, determinant of the inhibitory drive and of the coding performed by visual cortex. Recently, we have demonstrated a diurnal regulation of [Cl-]i that modulates the local field potential elicited by patterned visual stimuli (doi:10.1038/s41467-023-42711-7). However, the lack of a proper tool has limited the direct investigation of Cl- changes in response to sensory stimulations. Here we introduce iClima (improved Chloride Imaging): a novel ratiometric genetically encoded chloride sensor (GECS) to monitor the dynamics of [Cl-]i. GECSs are sensitive to intracellular pH since their affinity to Cl- depends on their protonation state (doi:10.1016/j.jneumeth.2021.109455). iClima was selected for its high affinity for protons (pKa= 7.8), so that fluctuations around physiological pH do not affect [Cl-]i measurement. We demonstrate that iClima excitation at 450nm (1-photon) or 830nm (2-photons) exhibits pH-independent fluorescence, yielding a direct readout of [Cl-]. Importantly, at physiological pH iClima exhibits approximately 10 times greater affinity for Cl- than common GECS, with a Kd of about 4.5 mM at pH 7.2. Remarkably, using 2-photon imaging of behaving mice we characterized chloride transients elicited in cell soma and dendrites by visual stimulation and found that the dynamics of these transients depend on the specific nature of the stimulus. These findings demonstrate a yet undiscovered spatio-temporal dynamics of the [Cl-]i landscape in vivo.