Piston linear actuator, in particular for a disc brake caliper (1, 2), with a reducer of the type comprising - a differential planetary reducer comprising at least two stages sharing a common kinematic element (PC2, 140, planet gears 141, or ring gear 1561), in which: ∘ a first planetary stage comprises a group of input planet gears (P2, 141, 241) which are driven at input by meshing with an input sun gear (S2, 139) and which mesh with a ring gear that is fixed in rotation (R2, 121), driving an input planet carrier (PC2, 140, 240) with a first reduction, and ∘ a second planetary stage (PR3, PR3b) comprises a group of output planet gears (P3, 141, 244) which are carried by an output planet carrier (140, 243) and which mesh with a ring gear (R3, 151, 251), to provide a second reduction (DR, PR2'); and - a screw and nut mechanism (SC-NU, 152-161, 252-251) driven in rotation by the output (R3, 151, 243) of said differential reducer (DPR, DPR') in order to displace a piston (16, 16b) that is moveable in translation along the axis of said differential planetary gearset, in the gripping direction, said actuator being characterized a) in that it also comprises an upstream reduction stage driving the differential reducer by an upstream planetary gearset comprising an upstream planet carrier (PC1, 130) rotationally integral with the input sun gear (S2, 139) of said differential reducer, said upstream planet carrier (PC1, 130) carrying a group of upstream planet gears (P1, 131) which: ∘ mesh with a fixed ring gear (R1, 121), and ∘ are driven at input by meshing with an upstream sun gear (S1, 111); and b) in that the output (R3, 151, 140) of the differential reducer drives in rotation a screw (SC, 152, 252) which cooperates with a nut (NU, 161, 251) which is fixed in rotation and which is formed inside a part that is fixed to the piston or is formed inside the piston (16, 16b) itself. - a differential planetary reducer comprising at least two stages sharing a common kinematic element (PC2, 140, planet gears 141, or ring gear 1561), in which: ∘ a first planetary stage comprises a group of input planet gears (P2, 141, 241) which are driven at input by meshing with an input sun gear (S2, 139) and which mesh with a ring gear that is fixed in rotation (R2, 121), driving an input planet carrier (PC2, 140, 240) with a first reduction, and ∘ a second planetary stage (PR3, PR3b) comprises a group of output planet gears (P3, 141, 244) which are carried by an output planet carrier (140, 243) and which mesh with a ring gear (R3, 151, 251), to provide a second reduction (DR, PR2'); and ∘ a first planetary stage comprises a group of input planet gears (P2, 141, 241) which are driven at input by meshing with an input sun gear (S2, 139) and which mesh with a ring gear that is fixed in rotation (R2, 121), driving an input planet carrier (PC2, 140, 240) with a first reduction, and ∘ a second planetary stage (PR3, PR3b) comprises a group of output planet gears (P3, 141, 244) which are carried by an output planet carrier (140, 243) and which mesh with a ring gear (R3, 151, 251), to provide a second reduction (DR, PR2'); and - a screw and nut mechanism (SC-NU, 152-161, 252-251) driven in rotation by the output (R3, 151, 243) of said differential reducer (DPR, DPR') in order to displace a piston (16, 16b) that is moveable in translation along the axis of said differential planetary gearset, in the gripping direction, a) in that it also comprises an upstream reduction stage driving the differential reducer by an upstream planetary gearset comprising an upstream planet carrier (PC1, 130) rotationally integral with the input sun gear (S2, 139) of said differential reducer, said upstream planet carrier (PC1, 130) carrying a group of upstream planet gears (P1, 131) which: ∘ mesh with a fixed ring gear (R1, 121), and ∘ are driven at input by meshing with an upstream sun gear (S1, 111); and ∘ mesh with a fixed ring gear (R1, 121), and ∘ are driven at input by meshing with an upstream sun gear (S1, 111); and b) in that the output (R3, 151, 140) of the differential reducer drives in rotation a screw (SC, 152, 252) which cooperates with a nut (NU, 161, 251) which is fixed in rotation and which is formed inside a part that is fixed to the piston or is formed inside the piston (16, 16b) itself.