Nitrobindins (Nbs), constituting a heme‐protein family spanning from bacteria to
Homo sapiens
, display an all‐β‐barrel structural organization. Human Nb has been described as a domain of the nuclear protein named
THAP
4, whose physiological function is still unknown. We report the first evidence of the heme‐Fe(
III
)‐based detoxification of peroxynitrite by the all‐β‐barrel
C
‐terminal Nb‐like domain of
THAP
4. Ferric human Nb (Nb(
III
)) catalyzes the conversion of peroxynitrite to
and impairs the nitration of free
l
‐tyrosine. The rate of human Nb(
III
)‐mediated scavenging of peroxynitrite is similar to those of all‐α‐helical horse heart and sperm whale myoglobin and human hemoglobin, generally taken as the prototypes of all‐α‐helical heme‐proteins. The heme‐Fe(
III
) reactivity of all‐β‐barrel human Nb(
III
) and all‐α‐helical prototypical heme‐proteins possibly reflects the out‐to‐in‐plane transition of the heme‐Fe(
III
)‐atom preceding peroxynitrite binding. Human Nb(
III
) not only catalyzes the detoxification of peroxynitrite but also binds
NO
, possibly representing a target of reactive nitrogen species.