The self-assembly of didodecyldimethylammonium bromide (DDAB) in a protic ionic liquid, ethylammonium nitrate (EAN), in the high surfactant concentration regime is studied using five different experimental techniques. A thermoreversible first-order sponge (L(3)) to lamellar (L(α)) transition occurring at [DDAB] > 80 wt % was identified by (1) a sharp increase in the elastic and viscous moduli, (2) a transition peak recorded by differential scanning calorimetry, (3) formation of Maltese cross birefringence textures observed via polarizing optical microscopy, (4) a decrease in the interbilayer mean distance measured by small angle neutron scattering, and (5) an abrupt increase in the conductivity obstruction factor. In contrast to aqueous DDAB solutions, this surfactant forms a stable L(3) phase in EAN in a wide window of compositions and temperatures, which is potentially useful for the synthesis of nanoporous materials. To the best of our knowledge, this is the first evidence of the formation of the L(3) phase in an ionic liquid.