Abstract Enzyme-catalyzed iterative β -1,4-glycosylation of β -glycosides is promising for bottom-up polymerization of reducing-end-modified cello-oligosaccharide chains. Self-assembly of the chains from solution yields crystalline nanocellulose materials with properties that are tunable by the glycoside group used. Cellulose chains with a reducing-end thiol group are of interest to install a controllable pattern of site-selective modifications into the nanocellulose material. Selection of the polymerizing enzyme (cellodextrin phosphorylase; CdP) was pursued here to enhance the synthetic precision of β -1-thio-glucose conversion to generate pure “1-thio-cellulose” (≥95%) unencumbered by plain (unlabeled) cellulose resulting from enzymatic side reactions. The CdP from Clostridium stercorarium ( Cs CdP) was 21 times more active on β -1-thio-glucose (0.17 U/mg; 45 °C) than the CdP from Clostridium cellulosi ( Cc CdP), and it lacked hydrolase activity, which is substantial in Cc CdP, against the α- d -glucose 1-phosphate donor substrate. The combination of these enzyme properties indicated that Cs CdP is a practical catalyst for 1-thio-cellulose synthesis directly from β -1-thio-glucose (8 h; 25 mol% yield) that does not require a second enzyme (cellobiose phosphorylase), which was essential when using the less selective Cc CdP. The 1-thio-cellulose chains had an average degree of polymerization of ∼10 and were assembled into highly crystalline cellulose II crystallinity material.