This study aimed to determine the effect of different levels of phosphorus doses on the nutrient concentrations, forage yield, and quality of Berseem (Trifolium alexandrinum L.) cultivar under semi-arid conditions. The research was conducted during the 2023-2024 growing season at the Research and Application Field of the Faculty of Agriculture, Siirt University, Siirt. The Derya cultivar of the Berseem was used as the plant material. Field trials were designed according to a randomized block design with four replications focusing on six different phosphorus doses (Po= 0.0 kg P ha-1, P3= 0.3 kg P ha-1, P6= 0.6 kg P ha-1, P9= 0.9 kg P ha-1,  P12= 1.2 kg P ha-1 ve P15= 1.5 kg P ha-1). The parameters examined included plant height, main stem thickness, green forage yield, dry matter yield, crude protein (CP), acid detergent fiber (ADF) ratio, neutral detergent fiber (NDF) ratio, relative feed value (RFV), as well as mineral concentrations of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), Ca/P ratio, and K/(Ca+Mg) ratios. The findings indicated that with increasing applications of phosphorus fertilizer, both plant height and main stem thickness, along with green and dry forage yields, demonstrated significant enhancement. The highest results were obtained by applying 0.9 kg of pure P per hectare for forage production. It was concluded that phosphorus fertilizer applications did not affect the quality of green forage in Berseem. Regarding the mineral content of Berseem, it was found that phosphorus fertilization did not create any differences in nutrient concentrations, such as P, Ca, and Mg, according to the doses. However, for potassium (K), negative effects were observed beginning from the application of 1.5 kg of P per hectare.