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This paper reports octave tunable dielectric combline bandpass filters with constant absolute bandwidth (CABW) using a novel re-entrant cap tuning technique. The resonant frequency is tuned by the hollow re-entrant cap penetrating into the filter cavity as an envelope around the dielectric resonator. This mechanism of tuning provides wider tuning capabilities and better spurious performance than the conventional screw-based tuning. Also, the cap tuners can be employed effectively to tune the input-output and inter-resonator couplings simultaneously with the frequency reconfiguration, enabling a CABW over a wide frequency tuning window. For proof of concept purposes, a single widely tunable resonator is presented with octave tuning ratio of 2.64:1, high quality factor from 1705 to 5480, and spurious-free band up to 3.44⋅ f0 . Afterwards, two octave tunable re-entrant cap filters are designed, fabricated, and tested. The first filter is a 78% widely tunable two-pole filter with a CABW of 43.5 ± 12% MHz, low insertion loss equals to 0.28 ± 0.03 dB, and a compact volume of 39 cm 3 . The second design is a four-pole octave tunable bandpass filter from 2.96 GHz to 1.36 GHz with a constant 69 ± 13% MHz bandwidth, low insertion loss better than 0.6 dB, return loss higher than 16 dB, and a compact 62 cm 3 structure. According to our own knowledge, thanks to the proposed tuning mechanism, the presented designs are the first CABW octave tunable high Q waveguide-based filters, having the widest tuning ranges over all similar state-of-the-art-designs.