We report on the generation of a narrow divergence (
θ
γ
<
2.5
 
 
mrad
), multi-MeV (
E
max
≈
18
 
 
MeV
) and ultrahigh peak brilliance (
>
1.8
×
10
20
 
 
photons
 
s
−
1
 
mm
−
2
 
 
mrad
−
2
0.1% BW)
γ
-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude
a
0
≈
2
). The spectrum of the generated
γ
-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.