Recently, metformin has been repurposed as a promising anticancer drug, but its cellular uptake mechanism in cancer cells is elusive. The need for useful bioimaging tools for metformin largely limits this. Here, we report an Ir(III) solvento complex as an imaging probe to detect metformin in aqueous buffer and in cellulo. This probe showed significant luminescence enhancement in the presence of metformin in aqueous buffer, with an LOD of 0.049 μM. Mechanistically, the acetonitrile ligands of the probe were replaced by metformin to achieve a “turn on” luminescence response. In HeLa cells, complex 1 showed a clear luminescence enhancement with increasing concentration of metformin. Adding an organic cation transporter 1 (OCT1) inhibitor largely reduced the cellular luminescence of the probe, demonstrating that OCT1 plays an important role in cellular uptake of metformin. To the best of our knowledge, this is the first bioimaging probe for visualizing metformin and its cellular uptake behavior in cancer cells. We anticipate that this probe could serve as a valuable tool for dissecting the mechanism of action of metformin in cancer cells in the future.