  We discuss black hole thermodynamics in the manifestly duality invariant
formalism of double field theory (DFT). We reformulate and prove the first law
of black hole thermodynamics in DFT, using the covariant phase space approach.
After splitting the full O(D, D) invariant DFT into a Kaluza-Klein-inspired
form where only n coordinates are doubled, our results provide explicit duality
invariant mass and entropy formulas. We illustrate how this works by discussing
the black fundamental string solution and its T-duals.
