An undersea liquid/gas separator device for separating liquid and gaseous phases of a fluid, in particular liquid and gaseous phases of crude oil, the device being characterized in that it comprises a structure capable of resting on the sea bottom and supporting the following pipes and manifolds suitable for being disposed as follows: a) at least one and preferably a plurality of first separator pipes extending from a horizontally-extending cylindrical first tank of elongate shape and circular section, the first separator pipes being connected thereto at their upstream ends, said first tank preferably being of diameter greater than the diameter of said first separator pipes, said first separator pipes being inclined at an angle α lying in the range 1° to 5° relative to the horizontal with a downward slope from said first tank towards a horizontal first cylindrical manifold to which said first pipes are connected downstream, said first diffuser tank being fed by at least one fluid feed pipe, and said first manifold being connected to a first liquid evacuation pipe (3 2) of downward slope, and preferably including downstream an export pump suitable for drawing said degassed liquid from said first manifold; and b) at least one and preferably a plurality of second separator pipes disposed above said first pipes from and above said first tank with a rising slope from said first tank towards a horizontal second cylindrical manifold, said second separator pipes being inclined relative to the horizontal at an angle β greater than α, preferably less than 10°, said second manifold preferably being of diameter greater than the diameter of said second pipes and smaller than the diameter of said first tank, said second manifold being connected to a first gas evacuation pipe; and c) a plurality of vertical link third pipes between each said first separator pipe and each said second separator pipe; and d) a system of preferably hydraulic actuators enabling the inclination of said first and second separator pipes to be adjusted and enabling said first tank and said first and second manifolds (20a, 30a) to be positioned horizontally. a) at least one and preferably a plurality of first separator pipes extending from a horizontally-extending cylindrical first tank of elongate shape and circular section, the first separator pipes being connected thereto at their upstream ends, said first tank preferably being of diameter greater than the diameter of said first separator pipes, said first separator pipes being inclined at an angle α lying in the range 1° to 5° relative to the horizontal with a downward slope from said first tank towards a horizontal first cylindrical manifold to which said first pipes are connected downstream, said first diffuser tank being fed by at least one fluid feed pipe, and said first manifold being connected to a first liquid evacuation pipe (3 2) of downward slope, and preferably including downstream an export pump suitable for drawing said degassed liquid from said first manifold; and b) at least one and preferably a plurality of second separator pipes disposed above said first pipes from and above said first tank with a rising slope from said first tank towards a horizontal second cylindrical manifold, said second separator pipes being inclined relative to the horizontal at an angle β greater than α, preferably less than 10°, said second manifold preferably being of diameter greater than the diameter of said second pipes and smaller than the diameter of said first tank, said second manifold being connected to a first gas evacuation pipe; and c) a plurality of vertical link third pipes between each said first separator pipe and each said second separator pipe; and d) a system of preferably hydraulic actuators enabling the inclination of said first and second separator pipes to be adjusted and enabling said first tank and said first and second manifolds (20a, 30a) to be positioned horizontally.