  Ultra-light primordial black holes, with masses
$m_\mathrm{PBH}<10^9\mathrm{g}$, evaporate before big-bang nucleosynthesis and
can therefore not be directly constrained. They can however be so abundant that
they dominate the universe content for a transient period (before reheating the
universe via Hawking evaporation). If this happens, they support large
cosmological fluctuations at small scales, which in turn induce the production
of gravitational waves through second-order effects. Contrary to the primordial
black holes, those gravitational waves survive after evaporation, and can
therefore be used to constrain such scenarios. In this work, we show that for
induced gravitational waves not to lead to a backreaction problem, the relative
abundance of black holes at formation, denoted $ \Omega_\mathrm{PBH,f} $,
should be such that $ \Omega_\mathrm{PBH,f}
<10^{-4}(m_\mathrm{PBH}/10^9\mathrm{g})^{-1/4}$. In particular, scenarios where
primordial black holes dominate right upon their formation time are all
excluded (given that $m_\mathrm{PBH}>10\, \mathrm{g}$ for inflation to proceed
at $\rho^{1/4}<10^{16}\mathrm{GeV}$). This sets the first constraints on
ultra-light primordial black holes.
