Salmo trutta caspius is one of the migratory (anadromous) and endemic fish species of the Caspian Sea which exists in the rivers and has a great economic and commercial importance to the region. This species lives in the salty and brackish water and returns to freshwater to spawn. In this study, demographic parameters of Salmo trutta caspius are investigated. A total of 104 samples were collected from Anzali to Shafarud in the southwest part of the Caspian Sea in the fall of 2018, using commercial gill-nets. The total length, total weight and gender of specimens were determined: in terms of sex ratio; the female population was 1.88 times more than the male population. The mean length for males was 67.7±7.8 cm and for females it was 65.6±6.8 cm. The mean weight for males and females was 3.0±0.1 kg and 2.8±0.81 kg, respectively. The correlation coefficient for length-weight relationship was 0.95. The growth rate (K) and asymptotic length (Lꝏ) for both genders were calculated to be 0.37 and 69 cm, respectively. Estimates for natural mortality (M), fishing mortality (F), total mortality (Z), and the exploitation coefficient were 0.3, 1.34, 1.64, and 0.81, respectively. This study showed that the population of this species is young in the southwest Caspian region. With regards to the mortality rate and negative algometric growth, it could be concluded that overfishing of older individuals has caused a reduction in the size of its population.