On 8 September 2017, a great (Mw 8.2) normal faulting earthquake ruptured within the subducting Cocos Plate ~70 km landward from the Middle American Trench beneath the Tehuantepec gap. Iterative inversion and modeling of teleseismic and tsunami data and prediction of GPS displacements indicate that the steeply dipping rupture extended ~180 km to the northwest along strike toward the Oaxaca coast and from ~30 to 70 km in depth, with peak slip of ~13 m. The rupture likely broke through the entire lithosphere of the young subducted slab in response to downdip slab pull. The plate boundary region between the trench and the fault intersection with the megathrust appears to be frictionally coupled, influencing location of the detachment. Comparisons of the broadband body wave magnitude (mB) and moment‐scaled radiated energy (ER/M0) establish that intraslab earthquakes tend to be more energetic than interplate events, accounting for strong ground shaking observed for the 2017 event. Plain Language Summary A large earthquake ruptured in the subducting Cocos Plate that underthrusts Mexico and Central America offshore of Chiapas, in southern Mexico. Analysis of seismic waves, deepwater tsunami recordings, and onshore geodetic displacements establishes that the rupture was on a steeply dipping fault plane and that the slip extended across the entire underthrust lithosphere, partially detaching the deeper slab. The event is located beneath the continental shelf, and there is a narrow zone of the megathrust from the Middle American Trench to where this event reached the plate boundary that appears to have frictional coupling, which likely influenced the location of the slab detachment. The event radiated stronger short‐period seismic waves than typical of comparable size events on subduction zone plate boundaries, producing severe damage in Oaxaca and Chiapas. Key Points The 2017 Mw 8.2 Chiapas normal faulting earthquake beneath the Tehuantepec gap involved lithospheric‐transecting rupture of the thin Cocos slab Seismic, tsunami, and GPS data indicate that the rupture extended ~100 km unilaterally to the northwest along strike and from ~30 to 70 km in depth Relative to megathrust earthquakes, subduction zone intraslab faulting is more energetic, resulting in strong ground shaking for the 2017 event