Copper (Cu) is essential for many biological processes, but is toxic when present in excessive amounts. In this study, we provide evidence that Cu plays a crucial role in controlling tuberculosis. A
Mycobacterium tuberculosis
(
Mtb
) mutant lacking the outer membrane channel protein Rv1698 accumulated 100-fold more Cu and was more susceptible to Cu toxicity than WT
Mtb
. Similar phenotypes were observed for a
M. smegmatis
mutant lacking the homolog Ms3747, demonstrating that these mycobacterial copper transport proteins B (MctB) are essential for Cu resistance and maintenance of low intracellular Cu levels. Guinea pigs responded to infection with
Mtb
by increasing the Cu concentration in lung lesions. Loss of MctB resulted in a 1,000- and 100-fold reduced bacterial burden in lungs and lymph nodes, respectively, in guinea pigs infected with
Mtb
. In mice, the persistence defect of the
Mtb mctB
mutant was exacerbated by the addition of Cu to the diet. These experiments provide evidence that Cu is used by the mammalian host to control
Mtb
infection and that Cu resistance mechanisms are crucial for
Mtb
virulence. Importantly,
Mtb
is much more susceptible to Cu than other bacteria and is killed in vitro by Cu concentrations lower than those found in phagosomes of macrophages. Hence, this study reveals an Achilles heel of
Mtb
that might be a promising target for tuberculosis chemotherapy.