Nanocrystalline powders of TiO2 and TiO2:Cr (0.1–10 at.% Cr) obtained by flame spray synthesis (FSS), are used as starting materials for preparation of gas sensors. Characterization of nanopowders is carried out by thermogravimetry (TG), Brunauer–Emmett–Teller (BET), adsorption isotherms, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Gas sensing materials are calcined at 400 °C in a form of tablets, the morphology of which is similar to that of starting materials. The mass loss of nanopowders upon heating, as determined from its temperature profile in TG, is correlated with the specific surface area (SSA) obtained from BET measurements. High SSA exceeding 100 m2/g is inherently related to the enhanced decomposition of organic residua below 400 °C. XRD diffraction patterns indicate small crystallite sizes (6–27 nm) and the presence of both polymorphic forms: anatase and rutile, independently of the form of nanomaterials. SEM images demonstrate agglomeration of crystallites into spherical grains. Gas sensing characteristics of TiO2:Cr nanosensors upon interaction with H2 are recorded in a self-assembled experimental system. Detection of hydrogen is carried out over the concentration range of 50–3000 ppm at the temperatures extending from 200 to 400 °C. It is demonstrated that nanomaterials based on TiO2:Cr are attractive for ultimate sensor applications due to a substantial decrease in the operating temperature down to 210–250 °C. At a certain level of doping (of about 5 at.%) a reversal of the sensor response from that of n-type to that of p-type semiconductor is seen. This effect can be accounted for by the acceptor-type substitutional defects Cr′TiCr′Ti built into TiO2 lattice.