  We measure the mass function for a sample of 840 young star clusters with
ages between 10-300 Myr observed by the Panchromatic Hubble Andromeda Treasury
(PHAT) survey in M31. The data show clear evidence of a high-mass truncation:
only 15 clusters more massive than $10^4$ $M_{\odot}$ are observed, compared to
$\sim$100 expected for a canonical $M^{-2}$ pure power-law mass function with
the same total number of clusters above the catalog completeness limit.
Adopting a Schechter function parameterization, we fit a characteristic
truncation mass of $M_c = 8.5^{+2.8}_{-1.8} \times 10^3$ $M_{\odot}$. While
previous studies have measured cluster mass function truncations, the
characteristic truncation mass we measure is the lowest ever reported.
Combining this M31 measurement with previous results, we find that the cluster
mass function truncation correlates strongly with the characteristic star
formation rate surface density of the host galaxy, where $M_c \propto$ $\langle
\Sigma_{\mathrm{SFR}} \rangle^{\sim1.1}$. We also find evidence that suggests
the observed $M_c$-$\Sigma_{\mathrm{SFR}}$ relation also applies to globular
clusters, linking the two populations via a common formation pathway. If so,
globular cluster mass functions could be useful tools for constraining the star
formation properties of their progenitor host galaxies in the early Universe.
