  We calculate constraints on radiatively decaying neutrinos from the recent
detection of singly ionized helium in the diffuse intergalactic medium (IGM) at
$z \simeq 3.3$. We consider a model in which neutrinos predominantly decay into
invisible relativistic particles with a rate $\tau^{-1}$, and with a small
branching ratio into the radiative mode. To satisfy the observation of singly
ionized helium, which puts a lower bound on the number density of singly
ionized helium in the IGM, we show that: for $\tau \ga 10^{18} \,{\rm sec}(1
\,{\rm eV}/m_\nu^2)$, transition moment of neutrinos $\mu_{12}$ is constrained
to be $\le 4 \hbox{--}8 \times 10^{-17} \,\mu_{\sc B}$ for $ 110 \,{\rm eV} \la
m_\nu \la 10 \,{\rm keV}$. We compare this bound with other astrophysical and
cosmological bounds on radiatively decaying neutrinos.
