Alloying is one of the most promising routes for tuning the physicochemical properties of noble metal-based nanocatalysts and thus improving their (electro)catalytic performance. Despites numerous achievements, bimetallic and trimetallic nanoalloys have still been thoroughly studied for the past two decades. In this study, metalloid boron (B) was alloyed within palladium (Pd)-based nanocatalysts to promote the electrochemical ethanol oxidation reaction (EOR) in alkaline media. The optimum PdCuB nanocatalyst exhibited remarkable electrochemical EOR activity (5.83 A mgPd-1) and good operation stability (both cycling and chronoamperometric studies). Mechanistic studies in both pure KOH and a KOH/ethanol mixture attributed superior EOR performance to positive synergistic effects of B in Pd-based nanocatalysts that kinetically accelerated the removal of poisoning ethoxy intermediates (the rate-determining step of EOR). They included (i) an electronic effect that changed the electronic structure of Pd and thus weakened the adsorption of poisoning ethoxy intermediates, (ii) a bifunctional effect that facilitated the adsorption of OHads and thus kinetically accelerated the further oxidation of poisoning intermediates, and (iii) a structural effect in which smaller B interstitially inserted into Pd-based nanocrystals and thus suppressed the physical Ostwald ripening processes.