A medical device for separating a fluid, in particular for separating platelet-rich plasma from whole blood, comprising: - a first container for receiving the fluid to be treated a second container (30; 20) for receiving a first fraction of the fluid, and a third container (20; 30) for receiving a second fraction of the fluid, the containers (10, 20, 30) each having a first port or opening (10a, 20a, 30a) for fluid inlet and/or fluid outlet; - a first connection line having a first line branch with a first end that can be associated to the first port of the first container and at least one intermediate bifurcation for defining at least one second line branch and one third line branch with respective second ends (24, 31) of the first connection line, the second ends (24, 31) of the first connection line being associatable to the first port (30a; 20a) of the second container (30; 20) and to the first port (20a; 30a) of the third container (20; 30); - valve means (42, 45, 47a, 48a-48b) on the first connection line for enabling or preventing a flow of fluid through at least one from among the first line branch, the second line branch, and the third line branch, the valve means (42, 47a) preferably including at least one valve means on the first line branch, wherein the first container comprises: - a hollow container body having an elongated shape with a first end and a second end, the container body having the respective first port at the first end and a second port at the second end; - a plunger, which is associated in a movable way to the container body and has a plunger head that is slidably engaged in a fluid-tight way inside the container body, for defining thereby a fluid-collection chamber that is in communication with the first port, the plunger having a plunger stem that can be displaced through the second port of the container body for varying the position of the plunger head and thereby varying the volume of the fluid-collection chamber, the plunger stem being preferably associated in a releasable way to the plunger head, wherein the device has a modular structure comprising a plurality of modules (A, B, C), which are connected together in a separable way, for enabling separation thereof according to the operating step of use of the device, and that include at least one first module, which comprises at least the first container and a corresponding portion of the first connection line, and a second module, which comprises at least one between the second and third containers (30, 20) with a corresponding portion (40b, 40c) of the first connection line, wherein provided at least at the interface between two modules (A, B, C) of the modular structure are releasable connectors (43, 44), in particular hydraulic connectors, comprising at least two parts (43a-43b; 44a-44b) that can be coupled together, each of which belongs to a corresponding module (A, B, C). - a first container for receiving the fluid to be treated a second container (30; 20) for receiving a first fraction of the fluid, and a third container (20; 30) for receiving a second fraction of the fluid, the containers (10, 20, 30) each having a first port or opening (10a, 20a, 30a) for fluid inlet and/or fluid outlet; - a first connection line having a first line branch with a first end that can be associated to the first port of the first container and at least one intermediate bifurcation for defining at least one second line branch and one third line branch with respective second ends (24, 31) of the first connection line, the second ends (24, 31) of the first connection line being associatable to the first port (30a; 20a) of the second container (30; 20) and to the first port (20a; 30a) of the third container (20; 30); - valve means (42, 45, 47a, 48a-48b) on the first connection line for enabling or preventing a flow of fluid through at least one from among the first line branch, the second line branch, and the third line branch, the valve means (42, 47a) preferably including at least one valve means on the first line branch, - a hollow container body having an elongated shape with a first end and a second end, the container body having the respective first port at the first end and a second port at the second end; - a plunger, which is associated in a movable way to the container body and has a plunger head that is slidably engaged in a fluid-tight way inside the container body, for defining thereby a fluid-collection chamber that is in communication with the first port, the plunger having a plunger stem that can be displaced through the second port of the container body for varying the position of the plunger head and thereby varying the volume of the fluid-collection chamber, the plunger stem being preferably associated in a releasable way to the plunger head, wherein the device has a modular structure comprising a plurality of modules (A, B, C), which are connected together in a separable way, for enabling separation thereof according to the operating step of use of the device, and that include at least one first module, which comprises at least the first container and a corresponding portion of the first connection line, and a second module, which comprises at least one between the second and third containers (30, 20) with a corresponding portion (40b, 40c) of the first connection line, wherein provided at least at the interface between two modules (A, B, C) of the modular structure are releasable connectors (43, 44), in particular hydraulic connectors, comprising at least two parts (43a-43b; 44a-44b) that can be coupled together, each of which belongs to a corresponding module (A, B, C).