Abstract: A firearm magazine extender is installable onto a firearm magazine having a lower end portion from which magazine flanges project. Embodiments of the firearm magazine extender include a first enclosure piece, a second enclosure piece, and a laterally-interlocking interface formed between the first and second enclosure pieces. The first enclosure piece includes, in turn, laterally-flexible sidewalls spaced along a lateral axis of the firearm magazine extender, and retention slots formed in the laterally-flexible sidewalls and into which the magazine flanges engage when the first enclosure piece is fit onto the firearm magazine. The second enclosure piece is configured to matingly engage the first enclosure piece and cooperate therewith to enclose the lower end portion of the firearm magazine.