Fischer-Tropsch synthesis (FTS) has been considered a promising way for conversion of syngas (H2 + CO) into synthetic liquid fuels and/or high value-added chemicals. Iron-based catalysts are highly promising for FTS due to their high activity and low cost. In general, the as-prepared iron-based catalysts are inactive for FTS and barely activated in usual FTS conditions. Therefore, they must be pre-activated in CO or low-pressure syngas (≤ 0.3 MPa) to change the as-prepared catalysts into active iron carbide species prior to the reaction, which is disadvantageous considering operation efficiency and facility simplification. Ferrihydrite is a nanocrystalline iron-oxyhydroxide, which is abundantly present in nature and, also, can be synthesized in iron-containing aqueous environments. Due to its small crystallite size and high surface area, the ferrihydrite has high potential as a catalyst for iron-catalyzed reactions such as FTS. But, only few studies about ferrihydrite as a FTS catalyst has been performed. In this study, we report a novel characteristic of synthetic ferrihydrite as a catalyst for FTS. Even though no activation pre-treatment was carried out, the siliceous ferrihydrite-based catalysts promoted by Cu and K showed high catalytic performance as they were spontaneously activated during the initial period of reaction. The performance of the spontaneously activated catalysts was comparable to that of the pre-activated catalysts in terms of catalytic activity and even higher than that of the pre-activated catalysts in terms of C5+ selectivity. Furthermore, we successfully demonstrated the spontaneous activation of ferrihydrite-based FTS catalysts in a pilot-scale slurry bubble column reactor (5-15 bbl/d).