River sand is used as fine aggregate in concrete and cement mortar. The rapid expansion in construction activity witnessed in the country over the last decade has led to an incessant demand for river sand. Hence river sand is being excessively mined at the riverbeds and is leading to fast depletion of the precious natural resource. This presses the need for exploration of alternative materials which can be adopted as fine aggregate by construction industry to build concrete and masonry buildings. One possible candidate is coal bottom ash. Coal-based thermal power plants produce tonnes of coal bottom ash along with fly ash during power generation. Coal bottom ash is coarse particles settled at the bottom of cooling towers. The coarse nature of coal bottom ash particles can be harnessed and explored. The present study focuses on utilization of coal bottom ash as fine aggregate in cement mortars. River sand is fully and partially replaced by coal bottom ash. Five different proportions of river sand being replaced with coal bottom ash are adopted, namely 0, 25, 50, 75 and 100%. Performance of replacement of river sand with coal bottom ash is assessed in terms of particle size distribution (PSD) curves, workability and mechanical properties. Sieve analysis and mortar flow table test are conducted to assess the PSD curves and workability. Compression tests are conducted on cement mortar cubes (with different proportion of coal bottom ash) to determine compressive strength. Further, compression test is conducted on cement mortar cylinders in a displacement-controlled universal testing machine (UTM) to obtain the stress–strain curve and modulus of elasticity. The study found that river sand replaced with up to 50% coal bottom ash exhibited satisfactory performance as fine aggregate in cement mortar.