We previously showed in our 3rd communication that a base probably isomeric to rhoeadin (melting point 172-174°C) can be isolated from Papaver Rhoeas, which was thought to be the component responsible for the red color of rhoeadin with mineral acid. In this study, we obtained this base in very pure form with a melting point of 165-167°C and initially named it Rhoeas-Alkaloid A. Independently, Santavy isolated a base R-A from Papaver Rhoeas with a melting point of 159-161°C, which may be identical to our alkaloid. UV spectroscopy revealed no differences between Rhoeas-Alkaloid A (λmax1=241 nm, λmax2=292 nm) and rhoeadin (λmax1=241 nm, λmax2=291 nm). Elemental analysis confirmed the formula C₂₁H₂₁NO₆, supporting its identity as a rhoeadin isomer. IR spectroscopy showed characteristic differences compared to rhoeadin: the absence of a band at 2800 cm⁻¹ and intensity reversal in the 1480 and 1360 cm⁻¹ region. Oxidation of Rhoeas-Alkaloid A with 2N nitric acid converted it to hydrastinin, confirming its benzyltetrahydroisoquinoline structure—consistent with rhoeadin. Thus, we propose the name isorhoeadin for this stereoisomer of rhoeadin. Rhoeas-Alkaloid A (isorhoeadin) is a third benzyltetrahydroisoquinoline alkaloid from Papaver Rhoeas, alongside rhoeadin and rhoeagenin.