The present invention relates to a dual-frequency resonant cavity (6) for cyclotron which includes a dee (10), a pillar (20), and a conducting enclosure (40) surrounding the pillar and the dee, an end of the pillar being connected to the base of the conducting enclosure and an opposite end of the pillar (20) supporting the dee (10). The conducting enclosure and the pillar form a transmission line comprising at least three portions (20a, 20b, 20c), each portion having a characteristic impedance (Zc1, Zc2, Zc3). The characteristic impedance Zc2 of the intermediate portion (20b) is substantially lower than the characteristic impedances Zc1 et Zc3 of the two other portions (20a, 20b), which makes it possible to have the cavity resonate according to two modes in order to produce two distinct frequencies, without having to make use of moving components such as for example sliding short-circuits or mobile plates.The present invention also relates to a method for designing such a resonant cavity, based on the use of electromagnetic—and radio frequency simulation tools.