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This paper reports octave tunable dielectric combline bandpass filters with constant absolute bandwidth (CABW) using a novel <italic>re-entrant cap</italic> 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 <inline-formula><tex-math notation="LaTeX">$3.44\cdot$</tex-math></inline-formula><inline-formula><tex-math notation="LaTeX">$f_\mathrm{0}$</tex-math></inline-formula>. Afterwards, two octave tunable re-entrant cap filters are designed, fabricated, and tested. The first filter is a 78&#x0025; widely tunable two-pole filter with a CABW of 43.5<inline-formula><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula>12&#x0025; MHz, low insertion loss equals to 0.28<inline-formula><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula>0.03 dB, and a compact volume of 39 cm³. The second design is a four-pole octave tunable bandpass filter from 2.96 GHz to 1.36 GHz with a constant 69<inline-formula><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula>13&#x0025; MHz bandwidth, low insertion loss better than 0.6 dB, return loss higher than 16 dB, and a compact 62 cm³ 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.