Horse-riding stirrup comprising a stirrup frame (2) which comprises a hooking portion (4) to a stirrup leather (100), a first side arm (6) and a support arm (8) connected to each other in an incident manner to define a frame compartment (10) engageable by a user's foot; a second side arm (12) comprising an articulation end (14) pivoted to the stirrup frame, and a free end (16) rotatable between a coupling configuration to the support arm and a release configuration therefrom in which the arm is spaced from the support arm to create a lateral disengagement opening (18) of the foot from the compartment. The second side arm comprises a flexible portion (20) which in the coupling configuration presses the free end in abutment with the support arm and which, in order to achieve the release configuration, allows the second side arm to bend in part to snap-disengage the free end from the support arm.