16H,18H-Dibenzo[c,l]-7,9-dithia-16,18-diazapentacene (DDDP), a new phenothiazine derivative containing two linearly condensed phenothiazine rings, strongly adsorbs on polyoriented gold resulting in a modified electrode with electrocatalytic activity for β-nicotinamide adenine dinucleotide (NADH) oxidation. Cyclic voltammetry (CV) measurements, performed in aqueous buffer solutions at different pH values, allowed to: (i) estimate the standard heterogeneous rate constants (k S ~1s - 1 , at pH 7.0) in the pH range between 4 and 8; (ii) demonstrate the electrocatalytic activity of the investigated compound for NADH oxidation. The second-order electrocatalytic rate constant k 1 was evaluated from the rotating disk electrode (RDE) technique (k 1 , [ N A D H ] = 0 =5.0x10 3 M - 1 s - 1 , at pH 7.0). The electrochemical behavior and electrocatalytic activity of DDDP are markedly influenced by the intramolecular interaction between the two conjugate phenothiazine rings. The electrocatalytic activity of DDDP (1e - , 1H + acceptor) was assigned to its ability to extract rapidly from NADH the first electron and the proton.