Lanthanoid luminescence has become an important pillar for many modern photonics applications such as bioanalytical research or functional material science. So far, however, thulium despite having one of the most interesting photophysics among the lanthanoids has suffered from extremely low luminescence efficiencies in molecular complexes with organic sensitizer ligands. This has greatly hampered the investigation and application of thulium emission in solution. Here, the discovery of a powerful sensitizer for thulium photoluminescence is reported. The corresponding thulium complex exhibits emission efficiencies (quantum yield Phi > 0.12%; lifetime tau(obs) = 4.6 mu s; brightness epsilon Phi > 30 M-1 cm(-1)) and can even be detected at low micromolar concentrations in high-phonon solvents like water without the need for laser excitation.