A novel twelve-channel three-dimensional (3-D) hyperthermia applicator has been developed and evaluated, which consists of twelve separate WAter COated Antenna (WACOA) modules. The modules are arranged in three transversal antenna rings (sub-arrays) and are placed into an acrylic applicator frame as cartridge-like elements in a staggered arrangement. The operating frequency is 100 MHz. For the design of the applicator, the finite-difference time-domain (FDTD) method was used. The applicator's dimensions allow its placement into the gantry of a magnetic resonance (MR) tomograph. The WACOA modules are designed as MR-compatible specially shaped metallic cylindrical dipole structures that are placed into hermetically closed water-filled cassettes. Due to the design of the dipole structures, only a conventional coaxial feed circuitry is needed, and no external impedance matching networks are necessary. Instead, fine on-line impedance matching is realized using adjustable tuning rods and matching rings, both elements being parts of the radiating antenna structure. Experimental and numerical evaluations demonstrate a good stability of impedance matching, a low inter-channel coupling of less than -20 dB, and a good ability of field pattern steering.