NiO nanowalls were grown by low-cost chemical bath deposition (CBD) method and thermal annealing. SEM analysis showed a high surface area and porous thin film formed by a network of ~20 nm thick and ~270 nm tall nanosheets. NiO nanowalls were prepared onto interdigitated electrodes and applied for acetone sensing. The response transients at various temperatures were recorded an successfully modelled by two adsorption sites. Both sites are active below 250 °C, while only the site with shorter response time is active above 250 °C. At the optimal temperature of 250 °C, the sensor demonstrated high sensitivity to acetone (1–40 ppm), a low limit of detection (LoD) of ~200 ppb and good selectivity. These unique features combined with the low-cost and simple fabrication process make it a cheap and efficient acetone sensor.