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Context. The XXL Survey is the largest survey carried out by the XMM-Newtonsatellite and covers a total area of 50 square degrees distributed over twofields. It primarily aims at investigating the large-scale structures of theUniverse using the distribution of galaxy clusters and active galactic nucleias tracers of the matter distribution.  Aims. This article presents the XXL bright cluster sample, a subsample of 100galaxy clusters selected from the full XXL catalogue by setting a lower limitof $3\times 10^{-14}\,\mathrm{erg \,s^{-1}cm^{-2}}$ on the source flux within a1$^{\prime}$ aperture.  Methods. The selection function was estimated using a mixture of Monte Carlosimulations and analytical recipes that closely reproduce the source selectionprocess. An extensive spectroscopic follow-up provided redshifts for 97 of the100 clusters. We derived accurate X-ray parameters for all the sources. Scalingrelations were self-consistently derived from the same sample in otherpublications of the series. On this basis, we study the number density,luminosity function, and spatial distribution of the sample.  Results. The bright cluster sample consists of systems with masses between$M_{500}=7\times 10^{13}$ and $3\times 10^{14} M_\odot$, mostly located between$z=0.1$ and 0.5. The observed sky density of clusters is slightly below thepredictions from the WMAP9 model, and significantly below the predictions fromthe Planck 2015 cosmology. In general, within the current uncertainties of thecluster mass calibration, models with higher values of $\sigma_8$ and/or$\Omega_m$ appear more difficult to accommodate. We provide tight constraintson the cluster differential luminosity function and find no hint of evolutionout to $z\sim1$. We also find strong evidence for the presence of large-scalestructures in the XXL bright cluster sample and identify five newsuperclusters.

The XXL Survey. II. The bright cluster sample: catalogue and luminosity function