The catalytic etherification of 5‐hydroxymethylfurfural (HMF) with t ‐butanol to a biofuel additive 5‐( t ‐butoxymethyl) furfural ( t ‐BMF) is studied by screening 27 heterogeneous protic and/or Lewis acids. Preyssler heteropolyacid H 14 [NaP 5 W 30 O 110 ] and Montmorillonite‐based CLOI_CSP catalysts are identified as the most efficient ones, allowing the etherification to reach the t ‐BMF/HMF equilibrium faster (≈1:1), thus promoting minimum side reactions. Further optimization of the batch experimental conditions, namely catalyst loading, temperature, HMF concentration, time, t ‐BMF stability, and additives, allows the t ‐BMF/HMF equilibrium to be reached in less than 3 h with outstanding selectivity >95%. Under optimized conditions, CLOI_CSP is reused for four cycles without significant loss of efficiency. In addition, it exhibits high efficiency under flow conditions, allowing continuous production of t ‐BMF up to 0.7 g t ‐BMF g −1 catalyst min −1 at high concentrations of HMF.