•We fabricate a metal–insulator–metal capacitive humidity sensor.•The functional sensing polymer comprises TiO2 nanoparticles.•The polymer is subjected to inductive plasma and reactive-ion etching.•This increases the sensor sensitivity, stability, and performance.•Our sensor is useful for temperature-independent applications. We propose a high-performance porous metal–insulator–metal-type capacitive humidity sensor based on a functional polymer mixed with TiO2 microparticles subjected to inductively coupled plasma (ICP) and reactive-ion etching (RIE) treatments. The humidity sensor is composed of a porous top electrode, a TiO2-containing functional polymer humidity-sensitive layer subjected to two types of oxygen plasma treatment, a bottom electrode, and a glass substrate. The initial O2 ICP dry-etching utilizes higher intensity plasma for deep etching in the inlet holes on the top electrode to increase the contact area and shorten the vapor absorption path, thereby yielding high sensitivity and low hysteresis. Further, the RIE treatment leads to roughening of the polymer etching surface and further improving the performance of the humidity sensor. The functional polymer mixed with TiO2 microparticles exhibits excellent hysteresis over a wide humidity sensing range. The fabricated sensors are tested at various relative humidity (RH) values, achieving an ultra-low hysteresis of 0.64% RH at 60% RH, a high sensitivity of 1.24pF/% RH, a fast response time of less than 25s, good temperature dependence, and a stable capacitance value with a maximum error rate of 0.15% over 120h of continuous testing.