We report the discovery of an L dwarf companion to the A3V star beta{} Circini. VVV J151721.49-585131.5, or beta{} Cir B, was identified in a proper motion and parallax catalogue of the Vista Variables in the V{i}a L{a}ctea survey as having near infrared luminosity and colour indicative of an early L dwarf, and a proper motion and parallax consistent with that of beta{} Cir. The projected separation of $sim$3.6 corresponds to $6656$ au, which is unusually wide. The most recent published estimate of the age of the primary combined with our own estimate based on newer isochrones yields an age of $370-500$ Myr. The system therefore serves as a useful benchmark at an age greater than that of the Pleiades brown dwarfs and most other young L dwarf benchmarks. We have obtained a medium resolution echelle spectrum of the companion which indicates a spectral type of L1.0$pm$0.5 and lacks the typical signatures of low surface gravity seen in younger brown dwarfs. This suggests that signs of low surface gravity disappear from the spectra of early L dwarfs by an age of $sim370-500$ Myr, as expected from theoretical isochrones. The mass of beta{} Cir B is estimated from the BHAC15 isochrones as $0.056pm0.007$ M$_{odot}$.
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