Mycobacterium tuberculosis is known to express a low-molecular weight protein tyrosine phosphatase. This enzyme, denoted as MptpA (Mycobacterium protein tyrosine phosphatase A), is essential for the pathogen to escape the host immune system and therefore represents a target for the search of antituberculosis drugs. MptpA was shown to undergo a conformational transition during catalysis, leading to the closure of the active site, which is by this means poised to the chemical step of dephosphorylation. Here we show that MptpA is subjected to substrate activation, triggered by p-nitrophenyl phosphate or by phosphotyrosine. Moreover, we show that the enzyme is also activated by phosphoserine, with serine being ineffective in enhancing MptpA activity. In addition, we performed assays under pre-steady-state conditions, and we show here that substrate activation is kinetically coupled to the closure of the active site. Surprisingly, when phosphotyrosine was used as a substrate, MptpA did not obey Michealis-Menten kinetics, but we observed a sigmoidal dependence of the reaction velocity on substrate concentration, suggesting the presence of an allosteric activating site in MptpA. This site could represent a promising target for the screening of MptpA inhibitors exerting their action independently of the binding to the enzyme active site.