Abstract: A holster is for a firearm and may include a holster body having a proximal wall to be adjacent a user, a distal wall opposite the proximal wall and defining a cavity to receive the firearm, a first longitudinal side coupling the proximal wall and the distal wall, and a second longitudinal side opposite the first longitudinal side and coupling the proximal wall and the distal wall. The holster may include a first retention device having an anchor body coupled between the proximal wall and the distal wall at the second longitudinal side, a flexible arm extending from the anchor body and to press against the firearm when in the cavity, and an adjustable feature carried by the anchor body and configured to bias the flexible arm against the firearm.