The present invention relates to the injection of water and gas, either simultaneously or alternately, into subsea wells in order to enhance production and the oil recovery level. Thus, the present invention provides a subsea system for pressurizing a subsea oil reservoir injecting at least one of water and gas, comprising (i) at least two subsea injection wells (4, 4′), each subsea injection well (4, 4′) being connected to a production unit (5) by means of a single subsea line (1, 2) which is connected to the respective subsea injection well (4, 4′) by a main injection mandrel (6, 6′), and (ii) at least one jumper (3), each jumper (3) hydraulically connecting two of the at least two adjacent subsea injection wells (4, 4′) through annular mandrels (7, 7′). The present invention further provides a subsea reservoir pressurization method associated with the above-described system.