An automatic fluid injection device comprising: a base body (1) for coming into contact with an injection zone; a support assembly (2) for supporting an actuator mechanism (5, 6, 7) that is controlled by power supply means (11); at least one fluid reservoir (3), each containing an injection piston (35), arranged in said base body (1); a needle assembly (100) comprising an insertion actuator (8), needle movement means (9, 9′), a priming needle (101) for associating with each reservoir (3) and for penetrating into said reservoir (3) before moving its piston (35), and an injection needle (102) for penetrating into the injection zone and for injecting the contents of said reservoir(s) (3) into said injection zone, said device including at least one actuator button (200) for priming, inserting the injection needle into the injection zone, administering fluid, and then retracting the injection needle.