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We study the constraint on the inflationary energy scale from the recentlyproposed Trans-Planckian Censorship Conjecture (TCC). We find a universal upperbound on the inflationary Hubble expansion rate $H_\text{inf}$ which is solelydetermined by the reheating temperature $T_\text{rh}$:$H_\text{inf}/M_\text{Pl}\lesssim T_0/T_\text{rh}$, where $T_0$ is the photontemperature today. The upper limit can be saturated by a post-inflationaryoscillatory stage with the critical equation-of-state parameter $w\approx-1/3$,or by an inflation model with multiple stages. In the lowest reheatingtemperature required for big bang nucleosynthesis, the upper bound on thetensor-to-scalar ratio $r$ at the CMB scales is $r\lesssim10^{-8}$, which canbe realized in many string-inspired inflation models.

Universal Upper Bound on the Inflationary Energy Scale from the Trans-Planckian Censorship Conjecture