  We report on the observation of previously-unknown isotope $^{21}$Al, the
first unbound aluminum isotope located beyond the proton dripline. The
$^{21}$Al nucleus decays by one-proton (1p) emission, and its in-flight decays
were detected by tracking trajectories of all decay products with micro-strip
silicon detectors. The 1p-emission processes were studied by analyses of the
measured angular correlations of decay products $^{20}$Mg+p. The 1p-decay
energies of ground and low-lying excited states of $^{21}$Al, its mass excess
and proton separation energy value $S_p$=$-1.1(1)$ MeV were determined.
