Carbon 1s core‐hole excitation of the molecular anion C2− has been experimentally studied at high resolution by employing the photon‐ion merged‐beams technique at a synchrotron light source. The experimental cross section for photo–double‐detachment shows a pronounced vibrational structure associated with 1σu→3σg ${1\sigma _u \to 3\sigma _g }$ and 1σg→1πu ${1\sigma _g \to 1\pi _u }$ core excitations of the C2− ground level and first excited level, respectively. A detailed Franck‐Condon analysis reveals a strong contraction of the C2− molecular anion by 0.2 Å upon this core photoexcitation. The associated change of the molecule's moment of inertia leads to a noticeable rotational broadening of the observed vibrational spectral features. This broadening is accounted for in the present analysis which provides the spectroscopic parameters of the C2− 1σu-13σg22Σu+ ${1\sigma _u^{ - 1} \,3\sigma _g^2 \;^2 \Sigma _u^ + }$ and 1σg-13σg22Σg+ ${1\sigma _g^{ - 1} \,3\sigma _g^2 \;^2 \Sigma _g^ + }$ core‐excited levels. Strong contraction: The C2− molecular anion shrinks when it absorbs an X‐ray photon.