A search is performed for the as yet unobserved baryonic Λ b 0 → Λη ′ and Λ b 0 → Λη decays with 3 fb−1 of proton-proton collision data recorded by the LHCb experiment. The B0 → K s 0 η ′ decay is used as a normalisation channel. No significant signal is observed for the Λ b 0 → Λη ′ decay. An upper limit is found on the branching fraction of $$ \mathrm{\mathcal{B}}\left({\Lambda}_{\mathrm{b}}^0\to \Lambda \upeta^{\prime}\right) $$ < 3.1 × 10−6 at 90% confidence level. Evidence is seen for the presence of the Λ b 0 → Λη decay at the level of 3σ significance, with a branching fraction $$ \mathrm{\mathcal{B}}\left({\Lambda}_{\mathrm{b}}^0\to \Lambda \upeta \right) = \left(9.{3}_{-5.3}^{+7.3}\right)\times 1{0}^{-6} $$ .