  We propose a search for $B$ meson decays to a baryon plus missing energy at
the Belle II experiment to probe supersymmetry with a GeV-scale lightest
neutralino ${\tilde{\chi}_1^0}$ and $R$-parity violation (RPV). We perform
analytic computations of the signal branching fractions in the framework of
effective field theory, with a single nonzero RPV operator
$\lambda''_{ij3}\bar{U}_{i}^c\bar{D}_{j}^c\bar{D}_3^c$, where $i,j=1,2$. The
hadronic form factors are calculated using an SU(3) phenomenological Lagrangian
approach for the proton, as well as several hyperons and charmed baryons. Since
the decay of the neutralino is kinematically and CKM suppressed in this
theoretical scenario, it decays outside the detector and appears experimentally
only as missing energy. We detail the analysis techniques at the experimental
level and estimate the background in the ${B^+ \to p {\tilde{\chi}_1^0}}$
search using published results for $B^+\to K^+ \nu\bar\nu$. Our final
sensitivity plots are shown for both $\lambda''_{113}$ versus the squark mass
$m_{\tilde{q}}$ and $\lambda''_{113}/m^2_{\tilde{q}}$ versus the neutralino
mass $m_{{\tilde{\chi}_1^0}}$. We find that the search at Belle II could probe
$\lambda''_{113}/m^2_{\tilde{q}}$ down to the order of $10^{-8}$ GeV$^{-2}$ in
the kinematically allowed $m_{{\tilde{\chi}_1^0}}$ range. We also obtain
current limits on $\lambda''_{123}$ by recasting an existing search interpreted
as ${B^0 \to \Lambda^0 {\tilde{\chi}_1^0}}$, and comment about searches for
${B^+ \to \Sigma^+ {\tilde{\chi}_1^0}}$, ${B^0 \to \Sigma^0
{\tilde{\chi}_1^0}}$, ${B^+ \to \Lambda_c^+ {\tilde{\chi}_1^0}}$, and ${B^+ \to
\Xi_c^+ {\tilde{\chi}_1^0}}$. In closing, we briefly discuss potential searches
at the LHCb and BESIII experiments.
