  We present the first detection of ($Z$)-1,2-ethenediol, (CHOH)$_2$, the enol
form of glycolaldehyde, in the interstellar medium towards the G+0.693-0.027
molecular cloud located in the Galactic Center. We have derived a column
density of (1.8$\pm$0.1)$\times$10$^{13}$ cm$^{-2}$, which translates into a
molecular abundance with respect to molecular hydrogen of
1.3$\times$10$^{-10}$. The abundance ratio between glycolaldehyde and
($Z$)-1,2-ethenediol is $\sim$5.2. We discuss several viable formation routes
through chemical reactions from precursors such as HCO, H$_2$CO, CHOH or
CH$_2$CHOH. We also propose that this species might be an important precursor
in the formation of glyceraldehyde (HOCH$_2$CHOHCHO) in the interstellar medium
through combination with the hydroxymethylene (CHOH) radical.
