GRB 111209A is the longest lasting gamma-ray burst (GRB) detected by the Swift Gamma-Ray Burst Mission, observed on December 9, 2011.

[1] Its duration is longer than 7 hours, implying this event has a different kind of progenitor than normal long GRBs.

It was first proposed that the progenitor of this event was a blue supergiant star with low metallicity.

Later, it was also proposed that this event is the prototype of a new class of GRBs, ultra-long GRBs.

The GRB was associated with the magnetar-powered supernova 2011kl, an object of intermediate luminosity between conventional GRB supernovae and superluminous supernovae.