We have discovered a wide planetary-mass companion to the $beta$ Pic moving group member 2MASSJ02495639-0557352 (M6 VL-G) using CFHT/WIRCam astrometry from the Hawaii Infrared Parallax Program. In addition, Keck laser guide star adaptive optics aperture-masking interferometry shows that the host is itself a tight binary. Altogether, 2MASSJ0249-0557ABc is a bound triple system with an $11.6^{+1.0}_{-1.3}$ $M_{rm Jup}$ object separated by $1950pm200$ AU (40) from a relatively close ($2.17pm0.22$ AU, 0.04) pair of $48^{+12}_{-13}$ $M_{rm Jup}$ and $44^{+11}_{-14}$ $M_{rm Jup}$ objects. 2MASSJ0249-0557AB is one of the few ultracool binaries to be discovered in a young moving group and the first confirmed in the $beta$ Pic moving group ($22pm6$ Myr). The mass, absolute magnitudes, and spectral type of 2MASSJ0249-0557 c (L2 VL-G) are remarkably similar to those of the planet $beta$ Pic b (L2, $13.0^{+0.4}_{-0.3}$ $M_{rm Jup}$). We also find that the free-floating object 2MASSJ2208+2921 (L3 VL-G) is another possible $beta$ Pic moving group member with colors and absolute magnitudes similar to $beta$ Pic b and 2MASSJ0249-0557 c. $beta$ Pic b is the first directly imaged planet to have a twin, namely an object of comparable properties in the same stellar association. Such directly imaged objects provide a unique opportunity to measure atmospheric composition, variability, and rotation across different pathways of assembling planetary-mass objects from the same natal material.
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