The 6.5 Jiuzhaigou earthquake occurred on 8 August 2017 36 km west‐southwest of Yongle, Sichuan, China. We use both ascending and descending Interferometric Synthetic Aperture Radar (InSAR) data from Sentinel‐1 and coseismic offsets of four Global Positioning System sites to obtain the coseismic surface deformation field and invert for the fault geometry and slip distribution. Most slip of the left‐lateral strike‐slip earthquake occurred in the 3–10 km depth interval with a maximum slip of about 1 m and a large shallow slip deficit (SSD). An eight‐month InSAR time‐series analysis documents a lack of resolvable postseismic deformation, and inversions for the distribution of postseismic slip demonstrate the lack of shallow afterslip. We argue that the observations of a pronounced SSD and no early afterslip of the Jiuzhaigou earthquake are indicative of an immature fault and that all incipient young strike‐slip faults likely feature a SSD. We would expect a complex rupture geometry with distributed coseismic failure in the uppermost part of the brittle crust during the fault‐zone development. As faults mature, they straighten out, develop a localized fault‐zone core, and the SSD diminishes. By calculating the static Coulomb stress change and nine‐year viscoelastic stress change caused by the Wenchuan earthquake, we also show that the 2008 Wenchuan earthquake did not significantly affect the time of occurrence of the 2017 Jiuzhaigou earthquake.