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Tectonic research of the Tibetan Plateau has long focused on its deformation style and mechanisms. The 2008 Mw7.9 Wenchuan earthquake ruptured the Longmen Shan fault located at the eastern rim of the plateau and excited a viscoelastic response of the lithosphere. We infer a three‐dimensional (3D) rheological structure of eastern Tibet from modeling nine years of postseismic displacements observed by GPS. Our solution provides tight constraints on the lower‐crustal and upper‐mantle steady‐state viscosities of the Songpan‐Ganzi Terrane as (5.0 ± 0.7) × 1018 and (1.3 ± 0.3) × 1019 Pa s, respectively, consistent with a "jelly sandwich" model of Tibet, but not with some crustal channel flow models featuring much lower viscosities. The inferred lower‐crustal and upper‐mantle transient viscosities are (5.0 ± 1.3) × 1017 and (5.0 ± 1.5) × 1018 Pa s, respectively, suggesting nonlinear deformation mechanisms. The adjacent West Qinling and Sichuan blocks feature an order‐of‐magnitude higher rheological strength, which is consistent with the changes in the crustal material properties and interseismic deformation style across the East Kunlun‐Tazang and Longmen Shan faults. Our results enable us to propose a conceptual 3D tectonic deformation model, in which the eastward extrusion of Tibet is absorbed in the Songpan‐Ganzi crust mainly by E‐W shortening and N‐S extension, accommodated through faulting of conjugate strike‐slip faults in the upper crust and distributed shear in the lower crust. Plain Language Summary: One of the fundamental questions in Earth science is the deformation mechanism of the continents, which controls the topography of the land we live on, the distribution of the resources extracted to support our living, and the origins of natural hazards such as landslides and earthquakes. The Tibetan Plateau, arguably, is the best natural laboratory to study the deformation mechanism of the continents. In this study, we address two critically important and much debated problems in Tibetan Plateau dynamics; that is, the rheologic structure of the Tibetan lithosphere and how it deforms under tectonic loading. We use nine years of mostly unpublished three‐dimensional (3D) deformation data collected in the aftermath of the 2008 Mw7.9 Wenchuan earthquake to infer the rheological structure of the eastern Tibet lithosphere. Our results are consistent with a "jelly sandwich" model, with a relatively weak lower crust underlain by a stronger upper mantle. However, they do not support the very low viscosities envisioned by some of the channel flow models of lower crustal deformation in eastern Tibet. We also propose a conceptual 3D tectonic deformation model, in which the eastward extrusion of Tibet is absorbed in the Songpan‐Ganzi crust by E‐W shortening and N‐S extension, accommodated through conjugate faults in the upper crust and distributed shear in the lower crust. Key Points: A comprehensive GPS data set is provided to cover nine years of three‐dimensional (3D) postseismic displacements after the 2008 Wenchuan earthquakeGPS constrained eastern Tibet Lithospheric rheological structure agrees with a "jelly sandwich" model, not models of much lower viscositiesA conceptual 3D tectonic deformation model in eastern Tibet is proposed to characterize its dynamic deformation style and mechanisms [ABSTRACT FROM AUTHOR]