A fuel pump for a direct injection system provided with a common rail; the fuel pump comprises: a pumping chamber defined in a main body; a piston, which is slidingly mounted inside the pumping chamber to cyclically vary the volume of the pumping chamber; an intake channel, which originates from a wall of the pumping chamber; an intake valve, which is arranged along the intake channel; a delivery channel, which originates from a wall of the pumping chamber; a delivery valve, which is arranged along the delivery channel; a cylindrical containing seat, which is defined in the main body under the pumping chamber, has a diameter larger than the diameter of the pumping chamber, and is delimited, on the upper side, by an annular abutment, which has, on the outside, the diameter of the containing seat and, on the inside, the diameter of the pumping chamber; and a guide bushing, which is housed in the containing seat, is centrally provided with a through hole where the piston is slidingly arranged, and has an upper surface, which abuts against the annular abutment of the containing seat; wherein the upper surface of the guide bushing and the annular abutment of the containing seat are shaped so as to create, between them, a hydraulic sealing, which is formed at a central area of the annular abutment; and wherein the annular abutment of the containing seat has a step, which is centrally arranged and against which a central portion of the upper surface of the guide bushing rests; the fuel pump being characterized in that the upper surface of the guide bushing has a rounded and convex shape having an increasing height from outside to inside so that the upper surface is externally lower and internally higher. a pumping chamber defined in a main body; a piston, which is slidingly mounted inside the pumping chamber to cyclically vary the volume of the pumping chamber; an intake channel, which originates from a wall of the pumping chamber; an intake valve, which is arranged along the intake channel; a delivery channel, which originates from a wall of the pumping chamber; a delivery valve, which is arranged along the delivery channel; a cylindrical containing seat, which is defined in the main body under the pumping chamber, has a diameter larger than the diameter of the pumping chamber, and is delimited, on the upper side, by an annular abutment, which has, on the outside, the diameter of the containing seat and, on the inside, the diameter of the pumping chamber; and a guide bushing, which is housed in the containing seat, is centrally provided with a through hole where the piston is slidingly arranged, and has an upper surface, which abuts against the annular abutment of the containing seat; wherein the upper surface of the guide bushing and the annular abutment of the containing seat are shaped so as to create, between them, a hydraulic sealing, which is formed at a central area of the annular abutment; and wherein the annular abutment of the containing seat has a step, which is centrally arranged and against which a central portion of the upper surface of the guide bushing rests; the fuel pump being characterized in that the upper surface of the guide bushing has a rounded and convex shape having an increasing height from outside to inside so that the upper surface is externally lower and internally higher.