BslA is an amphiphilic protein that forms a highly hydrophobic coat around Bacillus subtilis biofilms, shielding the bacterial community from external aqueous solution. It has a unique structure featuring a distinct partition between hydrophilic and hydrophobic surfaces. This surface property is reminiscent of synthesized Janus colloids. By investigating the behavior of BslA variants at water-cyclohexane interfaces through a set of multiscale simulations informed by experimental data, we show that BslA indeed represents a biological example of an ellipsoidal Janus nanoparticle, whose surface interactions are, moreover, readily switchable. BslA contains a local conformational toggle, which controls its global affinity for, and orientation at, water oil interfaces. This adaptability, together with single-point mutations, enables the fine-tuning of its solvent and interfacial interactions, and suggests that BsIA could be a basis for biotechnological applications.