The title compound, 3-(diphenylamino)-4-ethoxycyclobut-3-ene-1,2-dione (6), was prepared by reaction of diphenylamine (2) with diethyl squarate (DES; 5) as part of our ongoing studies on monosquarate-amides. Following purification and recrystallisation, the product was isolated as a green crystalline solid. Its structure was established by spectroscopic methods, including FTIR, ¹H NMR, ¹³C NMR and HRMS, and was unambiguously confirmed by single-crystal X-ray diffraction. This work provides access to a previously unreported diphenylamino-substituted squaric acid derivative.