A long-period radio transient, ASKAP J1935+2148, was discovered with a 54-minute period and exhibits three distinct emission states: a bright pulse state with highly linearly polarized pulses lasting 10–50 seconds; a weak pulse state with highly circularly polarized pulses lasting ~370 milliseconds; and a quiescent state with no pulses. These states evolve over 8 months, suggesting physical changes in the emission region. The source is located near the magnetar SGR 1935+2154 and is within a supernova remnant. The source shows strong linear polarization and a rotation measure (RM) of +159.3 rad m⁻², consistent with the Galactic foreground and other pulsars. The source is not detectable in every observation, indicating intermittency or nulling. The radio properties challenge current understanding of neutron-star emission and evolution. The source is located in the pulsar death valley, where detectable radio signals are not expected. The source is not associated with a foreground star. The observed period and emission could be explained by a rotating magnetic white dwarf (MWD) emitting coherent radio emission. However, the source's properties suggest a neutron-star-like emission mechanism, with similarities to PSR J1107-5907. The source's radio emission is much larger than the inferred spin-down luminosity, suggesting alternative emission mechanisms. The source is likely an ultralong period magnetar or neutron star. Continued monitoring is needed to determine if additional periods are present and the possible existence of a companion star. The source's properties challenge current models of radio emission from neutron stars and white dwarfs.A long-period radio transient, ASKAP J1935+2148, was discovered with a 54-minute period and exhibits three distinct emission states: a bright pulse state with highly linearly polarized pulses lasting 10–50 seconds; a weak pulse state with highly circularly polarized pulses lasting ~370 milliseconds; and a quiescent state with no pulses. These states evolve over 8 months, suggesting physical changes in the emission region. The source is located near the magnetar SGR 1935+2154 and is within a supernova remnant. The source shows strong linear polarization and a rotation measure (RM) of +159.3 rad m⁻², consistent with the Galactic foreground and other pulsars. The source is not detectable in every observation, indicating intermittency or nulling. The radio properties challenge current understanding of neutron-star emission and evolution. The source is located in the pulsar death valley, where detectable radio signals are not expected. The source is not associated with a foreground star. The observed period and emission could be explained by a rotating magnetic white dwarf (MWD) emitting coherent radio emission. However, the source's properties suggest a neutron-star-like emission mechanism, with similarities to PSR J1107-5907. The source's radio emission is much larger than the inferred spin-down luminosity, suggesting alternative emission mechanisms. The source is likely an ultralong period magnetar or neutron star. Continued monitoring is needed to determine if additional periods are present and the possible existence of a companion star. The source's properties challenge current models of radio emission from neutron stars and white dwarfs.