Preoperative marking or tattooing is a clinical procedure conducted during laparoscopic surgeries in which gastrointestinal lesions are identified by injecting India ink. However, diffusion of the carbon particles from the ink leads to its spreading in the surrounding tissues, resulting in unnecessarily extensive tissue resectioning. In this study, we determined whether this diffusion could be prevented using a sodium alginate (Alg-Na) hydrogel. We observed that the use of a 1.75% Alg-Na solution could significantly prevent the diffusion of carbon particles into the tissues of mice after subcutaneously injecting them with India ink. Similar results were observed in resected porcine large intestines. A good correlation between the concentration of Alg-Na and tattooed area was observed, in which an increase in its concentration led to an increase in its viscosity and injection pressure. To examine the effect of the molecular weight of Alg-Na on the tattooing area, a solution of 2% Alg-Na was irradiated with an electron beam at varying electron doses. Since irradiation could reduce the molecular weight, viscosity, and injection pressure of Alg-Na, a good correlation between these parameters and the tattooed area was observed. We concluded that an adequately viscous Alg-Na hydrogel could be used for clear preoperative marking.