The assembly of FtsZ plays an important role in bacterial cell division. Mycobacterium tuberculosis FtsZ (MtbFtsZ) has a single cysteine residue at position 155. We have investigated the role of the lone cysteine residue in the assembly of MtbFtsZ using different complimentary approaches, namely chemical modification by a thiol-specific reagent 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) or a cysteine-chelating agent HgCl₂, and site-directed mutagenesis of the cysteine residue. HgCl₂ strongly reduced the polymerized mass of MtbFtsZ while it had no detectable effect on the polymerization of Escherichia coli FtsZ, which lacks a cysteine residue. HgCl₂ inhibited the protofilamentous assembly of MtbFtsZ and induced the aggregation of the protein. Further, HgCl₂ perturbed the secondary structure of MtbFtsZ and increased the binding of a hydrophobic probe 1-anilinonaphthalene-8-sulfonic acid (ANS) with MtbFtsZ, indicating that the binding of HgCl₂ altered the conformation of MtbFtsZ. Chemical modification of MtbFtsZ by DTNB also decreased the polymerized mass of MtbFtsZ. Further, the mutagenesis of Cys-155 to alanine caused a strong reduction in the assembly of MtbFtsZ. Under assembly conditions, the mutated protein formed aggregates instead of protofilaments. Far-UV CD spectroscopy and ANS binding suggested that the mutated MtbFtsZ has different conformation than that of the native MtbFtsZ. The effect of the mutation or chemical modification of Cys-155 on the MtbFtsZ assembly has been explained considering its location in the MtbFtsZ crystal structure. The results together suggest that the cysteine residue (Cys-155) of MtbFtsZ plays an important role in the assembly of MtbFtsZ into protofilaments.