Abstract: The present invention relates to a mobile electronics device (10) to be fastened to a human body part (200), comprising an electronics housing (20) having electronics components (30) situated therein, wherein a flexible fastening arm (40) is situated in each case on the electronics housing (20) on two oppositely oriented wall portions (22), which fastening arms extend away from the electronics housing (20) and are flexibly deformable between a fastening position (FP), fastened to the body part (200), and a release position (RP), releasing the body part (200), and wherein, in the release position (RP), the fastening arms (40) are preloaded towards the fastening position (FP) with a preloading force (PF).