Context. Hot Jupiters are among the most suitable targets for atmospheric studies. Expanding the number of hot gaseous giant planets with atmospheric characterisations can improve our understanding of the chemical-physical properties of their atmospheres, as well as the formation and evolution of these extreme planets. Aims. In this work, we use high-resolution spectroscopy in the near-infrared (NIR) to search for chemical signatures in the atmospheres of the two hot Jupiters KELT-8 b (Teq = 1675‑55+61 K) and KELT-23 Ab (Teq = 1561 ± 20 K) and present a first characterisation of their atmospheric properties. Methods. We measured the transmission spectrum of each target with the near-infrared (NIR) high-resolution spectrograph GIANO-B at the TNG. We searched for atmospheric signals by cross-correlating the data with synthetic transmission spectra. To characterise the chemical-physical properties of the atmospheres of both planets, we ran two different atmospheric retrievals for each dataset: a retrieval assuming chemical equilibrium and a 'free-chemistry' retrieval, in which the abundance of each molecule could vary freely. Results. We detect water vapour (H2O) in the atmospheres of both KELT-8 b and KELT-23 Ab with a signal-to-noise ratio of S/N = 6.6 and S/N = 4.2, respectively. The two retrievals indicate a water-rich atmosphere for both targets. In the case of KELT-8 b, we determine a water volume mixing ratio of log10 (VMRH2O) = ‑2.07‑0.72+0.53, a metallicity of [M/H = 0.77‑0.89+0.61 dex, and a sub-solar C/O ratio (C/O ≤0.30, at 2 σ). For KELT-23 Ab, we find log10 (VMRH2O) = ‑2.26‑1.24+0.75, [M/H]= ‑0.42‑1.35+1.56 dex, and C/O ratio ≤0.78 (at 2 σ). The constraints on the metallicity and C/O ratio are based on the assumption of chemical equilibrium. Comparing these atmospheric chemical properties with those of the host stars, we suggest that for both planets, the accretion of gaseous material occurred within the H2O snowline in a pebble-rich disk enriched in oxygen due to sublimation of water ice from the inward-drifting pebbles. Conclusions. We investigated the atmospheres of KELT-8 b and KELT-23 Ab for the first time, finding water vapour in both of them and placing first constraints on their properties. These two planets are promising targets for future high- and low-resolution observations.