Electromagnetically induced transparency (EIT) has been realized in atomic systems, but fulfilling the EIT conditions for artificial atoms made from superconducting circuits is a more difficult task.Here we report an experimental observation of the EIT in a tunable three-dimensional transmon by probing the cavity transmission. To fulfill the EIT conditions, we tune the transmon to adjust its damping rates by utilizing the effect of the cavity on the transmon states. From the experimental observations, we clearly identify the EIT and Autler-Townes splitting (ATS) regimes as well as the transition regime in between. Also, the experimental data demonstrate that the threshold ΩAIC determined by the Akaike information criterion can describe the EIT-ATS transition better than the threshold ΩEIT given by the EIT theory.the electromagnetic-pumping doublet structure in the absorption or transmission spectrum [21]. Because of the similar transparency windows in the spectrum, EIT has often been confused with ATS. In the field of superconducting quantum circuits, there have been some experiments [22][23][24][25][26][27][28] involving either ATS or EIT. Theoretical analyses indicated that the claimed EIT was actually ATS [29,30]. This is because of the difficulty for these superconducting circuits to satisfy the dampingrate conditions for realizing the EIT in experiments [30]. Therefore, it remains an unsolved, important problem to realize EIT in a superconducting quantum circuit.