Rare $b$-hadron decays mediated by $b \rightarrow s \ell^+\ell^-$ transitions provide a sensitive test of lepton flavour universality, a property of the Standard Model of particle physics which states that the coupling of the electroweak gauge bosons to all lepton flavours is equal. Extensions of the Standard Model do not necessarily preserve this symmetry and may give sizeable contributions to these processes. Precise measurements of lepton flavour universality ratios are, therefore, an extremely sensitive probe for new physics beyond the Standard Model. Recent results from LHCb on lepton flavour universality tests in rare $b \rightarrow s \ell^+\ell^-$ decays are discussed.