This paper presents the fabrication and characterization of radio frequency (RF) and microwave passive structures on an air substrate using additive manufacturing (3-D printing). The air substrate is realized by 3-D printing RF structures in two separate pieces and snapped together face to face using a LEGO-like process. Spacers printed on the periphery provide the desired air substrate thickness. Metal patterning on nonplanar printed plastic structures is carried out using a damascene-like process. Various RF structures such as low dispersion transmission line, T-line resonator, high-gain patch antenna, slot antenna, and cavity resonator are demonstrated using this process. Good performance is achieved; for example, measured 50-Ω transmission line shows low loss of 0.17 dB/cm at 4 GHz, and a patch antenna (center frequency of 4.5 GHz) shows gain and bandwidth of 7.6 dB and 0.2 GHz, respectively. Details of both measured and simulation results are presented.