Date palm leaves have been diachronically applied in building materials in the Middle East and North Africa (MENA) region, so as to enhance specific properties, such as volume stability and strength. This research work concerns an experimental study on the impact of date palm leaflet powder (DPLP) on the thermal and physico-mechanical properties of gypsum mortars. A series of mortar compositions were prepared with different proportions of DPLP (0, 1, 3, and 5% w/w of binder) and variant particle size (0.5, 1, and 1.5 mm). The results showed that the mortars containing DPLP exhibited significant changes in their properties due to variations in DPLP concentration and particle size. Increased DPLP led to lower density, higher porosity, and water absorption rate, whereas mechanical strength and thermal conductivity were decreased according to the DPLP proportion and size. This research provides valuable insights into the use of sustainable and renewable building materials, highlighting the benefits of exploiting agricultural waste in the constructional sector. The findings lay the groundwork for future research and innovation in environmentally friendly construction technologies.