We analyse the photometric, chemical, star formation history and structural properties of the brightest globular cluster (GC) in M81, referred as GC1 in this work, with the intention of establishing its nature and origin. We find that it is a metal-rich ([Fe/H]=−0.60 ± 0.10), alpha-enhanced ([α/Fe] ~ 0.20 ± 0.05), core-collapsed (core radius rc = 1.2 pc, tidal radius rt = 76rc), old (> 13 Gyr) cluster. It has an ultraviolet excess equivalent of ~ 2500 blue horizontal branch stars. It is detected in X-rays indicative of the presence of low-mass binaries. With a mass of 1.0 × 10⁷ Mʘ, the cluster is comparable in mass to M31-G1 and is four times more massive than ω Cen. The values of rc, absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.