Transparent ceramics (TCs) represent a new family of functional hard materials. In this Letter, steady-state and time-resolved upconversion photoluminescence in Yb3+-Er3+ co-doped TC of yttrium aluminum garnet (TC-YAG) are reported for the first time, to the best of our knowledge. Under the excitation of near-infrared 940 nm laser at room temperature, the Yb3+-Er3+ co-doped TC-YAG emits intense multi-color luminescence consisting of cyan, green, and red groups of sharp lines. More excitingly, the green group of luminescence due to the transitions from 4S3/2 to 4I15/2 states of Er3+ are the prominent components with the average lifetime of ∼0.3ms. The internal quantum efficiency of the green luminescence is estimated to be 32.8%. A unique dual-resonance energy transfer from Yb3+ to Er3+ via the excited-state vibronic transitions is proposed as the principal mechanism of the strongest green luminescence of Er3+ ions in TC-YAG.