  We study time-reversal~(T) asymmetries in the charmless two-body decays of
antitriplet bottom baryons. We find that in ${\bf B}_b \to {\bf B}_n P$ with
${\bf B}_b = (\Xi _b ^- , \,\Xi_b^0, \,\Lambda_b)$ and ${\bf B}_n(P)$ a octet
baryon (pseudoscalar meson), the positive and negative helicity amplitudes are
both sizable, resulting in the large T-violating effects. Particularly, the
T-odd parameters, $\beta_w$, for the color-enhanced channels are expected to be
around $40\%$ and $-25\%$ in $b\to s $ and $b\to d $ transitions for the
standard model, which can be measured by the experiments at LHCb, respectively.
On the other hand, in ${\bf B}_b \to {\bf B}_n V(\gamma)$ with $V(\gamma)$ a
vector meson~(photon), the decays are predominated by a single helicity
amplitude, and the T violating effects by the correlations among spins are
suppressed. We also explore the T violating observables for the $\Xi_b$ decays
based on the angular distributions.
