A search for the rare two-body charmless baryonic decay $B^+ to p barLambda$ is performed with $pp$ collision data, corresponding to an integrated luminosity of $3,mbox{fb}^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. An excess of $B^+ to p barLambda$ candidates with respect to background expectations is seen with a statistical significance of 4.1 standard deviations, and constitutes the first evidence for this decay. The branching fraction, measured using the $B^+ to K^0_{mathrm S} pi^+$ decay for normalisation, is begin{eqnarray} mathcal{B}(B^+ to p barLambda) & = & ( 2.4 ,^{+1.0}_{-0.8} pm 0.3 ) times 10^{-7} ,, onumber end{eqnarray} where the first uncertainty is statistical and the second systematic.
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