The rigid [Cu(dmp)(phanephos)]⁺ complex displays a high luminescence quantum yield of 80% at ambient temperature. In contrast to the long-lived phosphorescence of 240 μs at T < 120 K with a radiative rate of k_r = 3 × 10³ s⁻¹, the ambient-temperature emission represents a thermally activated delayed fluorescence (DF) with a decay time of only 14 μs and a radiative rate of k_r(DF) = 6 × 10⁴ s⁻¹. Evidence for the involvement of the excited singlet state in the emission process is presented. This material has high potential to be applied in efficient OLEDs taking advantage of the singlet harvesting mechanism.