A new approach to ratiometric luminescent thermometry based on dual emission with fluorescent and phosphorescent origins exhibited by mononuclear Cu(I) complexes with cyclic P2N2-ligands is reported. The studied copper(I) complexes with cyclic P2N2-ligands, containing biphenylphosphine substituents in the phosphorus atoms, demonstrate dual emission in the solid state, with two distinct bands corresponding to S1→S0 fluorescence (446 nm) and T1→S0 phosphorescence (491 nm). Temperature-dependent photophysical studies reveal different thermal behaviors for each emission band. Two ratiometric sensing strategies were applied: steady-state luminescence yielded a maximum thermal sensitivity of 2.02 % K−1 at 150 K, while time-resolved emission spectroscopy, using lifetime differences to separate the emission bands, extended the working range and improved the sensitivity up to 3.86 % K−1 (5–200 K). These results highlight the potential of mononuclear Cu(I) complexes exhibiting dual emission with fluorescent and phosphorescent origins as a new type of ratiometric temperature sensor with tunable photophysical properties.