Microglia, the innate immune cells of the central nervous system (CNS), act as first responders to brain injury. Their ability to switch between different neuroprotective and neurotoxic phenotypes, plays a central role in maintaining brain homeostasis. Recently, the P2Y12 receptor (P2Y12R) has been identified as a promising molecular biomarker for microglia activity, as its expression level is dependent on microglia phenotype and function. P2Y12R positron emission tomography (PET) might be a valuable diagnostic tool, however, tracers with sufficient brain retention have not been reported so far. Herein, we report a brain-permeable P2Y12R PET tracer for in vivo imaging of P2Y12R-positive microglia. Nicotinate [18F]12 exhibited nanomolar affinity and specificity for the target receptor and showed a reduced uptake in microglia-depleted (PLX) mice, in comparison to WT and Trem2 knockout (Trem2-/-) mice. Ex vivo immunohistochemistry (IHC) and PET data revealed a strong correlation between microglia abundance, P2Y12R expression levels and tracer uptake.