Rare $B$ decays and processes with the ATLAS detector

We present rare $B$ decays and processes measured with the ATLAS detector. First, the associated production of vector boson + prompt $J/\psi$ that is a key observable for understanding of quarkonium production mechanisms and second, the rare $B_s^0\to\mu^+\mu^-$ decay, that due to its small branching fraction, is an excellent probe for physics beyond the standard model.