This　paper　reports　on　a　new　class　of　broadband　fully　3-D　printed　E-plane　rectangular-coax-to-waveguide　transition.　This　type　of　transition　is　adopted　to　interconnect　two　E-plane　rectangular　waveguides,　and　is　based　on　a　full　air-filled　waveguide　structure　without　introducing　other　types　of　transmission　lines　(TLs).　It　exhibits　little　dielectric　loss　and　dispersion,　and　furthermore　a　potentially　enhanced　power　handling　capability.　The　transition　is　constructed　by　a　section　of　air-filled　rectangular　coaxial　TL　cascaded　in　series　to　a　broadband　coax-to-waveguide　probe　transition.　A　proof-of-concept　X-band　back-to-back　transition　is　designed　and　prototyped　monolithically　by　incorporating　stereolithography-based　3-D　printing　and　copper　electroplating　techniques.　The　RF-measured　result　demonstrates　a　broadband　and　low-loss　characteristic　of　the　transition-an　insertion　loss　of　averagely　0.5　dB　and　a　return　loss　of　mostly　better　than　15　dB　at　the　entire　X　band.