Bread wheat is one of the major global cereals, and staple crop in the Sultanate of Oman. However, sustainable wheat production is threatened by several challenges including salinity, drought, and high temperature stresses. Drought and heat stresses often occur together, and cause substantial reduction in grain yield. This study was conducted to evaluate the potential of nano chitosan-glycinebetaine seed priming in improving the tolerance against combined drought and heat stresses in two bread wheat (Triticum aestivum L.) genotypes Annaj-2017 and TW1509). For priming, the seeds of wheat genotypes were soaked in aerated distilled water (hydropriming) or nano-sized chitosan- glycinebetaine solution (100 mM) for 18 h. The seeds (six seeds per pot) were sown in plastic pots filled with Universal potting media (2.5 kg per pot) maintained at 70% water holding capacity (WHC). Two weeks after sowing, pots were divided in four groups viz. i. 70% WHC at 25/18± 2°C (control), ii. 35% WHC at 25/18± 2°C (drought stress), iii. 70% WHC at 37/28 ± 2°C (heat stress) and iv. 35% WHC at 37/28 ± 2°C (heat stress). Heat and drought stresses and their combinations caused significant reduction in plant growth and grain yield formation through disturbance in plant water relations, stress induced oxidative damages, stomatal alternations causing, decline in carbon influx and metabolic disruptions. However, the effect of combined heat and drought stresses was more devastating effects than individual heat and drought stresses. Seed priming with nano-sized chitosan-glycinebetaine improved the growth, grain development and yield formation in two bread what genotypes Annaj-2017 and TW1509 under drought and heat stresses and their combination. This improvement was attributed to accumulation of osmolytes, like glycinebetaine and proline, activation of antioxidant enzymes, maintenance of tissue water status which aided to scavenge the stress-induced oxidative damages and sustain carbon influx and assimilation and grain development. In conclusion, seed priming with nano-sized chitosan glycinebetaine improved tolerance against combined heat and drought stresses in wheat genotypes through osmotic adjustment, maintenance of tissue water status, activation of antioxidant defence system, continuation of carbon assimilation and activities of grain-filling enzymes which sustained plant growth and yield formation.