In order to explore the energy accumulation of somatic tissues and their impacts on reproductive investment in Argentine shortfin squid (Illex argentinus), one of the most important commercial cephalopod species in the world, the energy density of soma tissues including the mantle, head-arms and fins of the squid was determined using the technique of tissue energy density. The relative energy accumulation of each soma tissue was analyzed between sexes, and was also regressed on the squid's gonadosomatic index (GSI) to justify the potential sex-specific differences in the process of soma energy accumulation and allocation to reproduction. The results showed that the maximum energy density was detected in the fins for both female and male individuals, with an average of (21.32±1.13) and (21.56±0.97) kJ/g, respectively. The energy density of mantle tissue was determined (21.15±0.69) kJ/g for females and (21.13±0.79) kJ/g for males, respectively. For the head-arms, the energy density was detected at the minimum value, with an average of (20.64±0.73) kJ/g for females and (20.66±0.70) kJ/g for males, respectively. For the females, the relative energy accumulation was estimated an average of 51.37%±4.77% for mantle, 37.42%±4.44% for head-arms and 11.21%±1.62% for fins, respectively. The relative energy accumulation of mantle of male squids was estimated the maximum value, with an average of 46.80%±5.42%, followed by head-arms with an average of 42.67%±6.05% and fins with an average of 10.53%±2.03%. Significant differences were only found in the relative energy accumulation between maturity stages for the mantle and head-arms of female squid. In addition, the results of linear mixed-effects models showed that the relative energy accumulation of mantle and head-arms was significantly correlated with the GSI for the female squid, while there were not any significant relationships between the relative energy accumulation of each soma tissue and the GSI for the males. Cumulatively, these findings revealed that the fins exhibit the greatest energy density among the soma tissues, and the mantle accumulates the largest energy reserve among the soma tissues. The energy reserve of the mantle was probably mobilized to support gonadal development in female I. argentinus, contrasting to the fitness maintenance of soma tissues for the males.