In this work we set out to study the activity of a thermostable Transketolase (TK) from Geobacillus stearothermophilus (TKgst) in an ionic liquid as cosolvent, which has never been investigated before with this enzyme. 1-Butyl-3-methylimidazolium chloride ([BMIm][Cl]) in the range 30–50 % in water maintained the total activity of TKgst and increased the reaction rate in the presence of pentoses as acceptor substrates, particularly d-ribose. To improve the synthetic process, TKgst was immobilized on an inorganic support, layered double hydroxides (LDHs), with excellent immobilization yield and catalytic activity using a simple, eco-compatible, efficient coprecipitation procedure. The biohybrid MgAl@TKgst was tested in 30 % [BMIm][Cl] for the synthesis of a rare, very costly commercially available sugar, d-sedoheptulose, which was obtained in one step from d-ribose with an isolated yield of 82 %. This biohybrid was reusable over four cycles with no loss of enzymatic activity. The particular activity of free and immobilized TKgst in [BMIm][Cl] holds promise to extend the applications of TKgst in other ionic liquids and unusual media in biocatalysis.