An extension adapter of a plug-in coupling, with which a free space between a male coupling element and a female coupling element can be bridged in a connecting manner and which comprises the following features: a male coupling end which can be inserted into the female coupling element, a female coupling end in which the male coupling element can be received and which is connected to the male coupling end via an intermediate piece, and a locking element which is displaceably arranged in the axial direction of the extension adapter and with which, in a locking position surrounding the female coupling end, the release of a male coupling element from the female coupling end is prevented.