A magazine extension comprises an extension body having an open upper end configured to be slidably coupled with an open bottom end of a magazine tube for an ammunition magazine, a retaining member, and a base plate. The retaining member is removably receivable in a passage defined through a rear wall of the extension body to releasably secure the body to the magazine tube. A portion of the retaining member protrudes out of an upper end of the passage and overlaps a rear wall of the magazine tube to limit sliding movement of the tube relative to the extension body when the tube is coupled with the body and the retaining member is received in the passage. The base plate is configured to be slidably coupled with an open lower end of the extension body and obstruct a lower end of the passage to releasably secure the retaining member in the passage.