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We report the discovery of a potentially major supercluster that extends across the Galactic plane in the constellation of Vela, at a mean recessional velocity of ∼18 000 km s−1. Recent multiobject spectroscopic observations of this Vela supercluster (VSCL), using AAOmega+2dF and the Southern African Large Telescope, confirm an extended galaxy overdensity in the Zone of Avoidance (ZOA) located where residual bulk flows predict a considerable mass excess. We present a preliminary analysis of ∼4500 new spectroscopic galaxy redshifts obtained in the ZOA centred on the Vela region (l = 272.°5±20°, b=0° ± 10°). The presently sparsely sampled data set traces an overdensity that covers 25° in Galactic longitude on either side of the Galactic plane, suggesting an extent of 25 × 20 deg2, corresponding to ∼ 115 × 90 h70 Mpc at the supercluster redshift. In redshift space, the overdensity appears to consist of two merging wall-like structures, interspersed with clusters and groups. Both the velocity histogram and the morphology of the multibranching wall structure are consistent with a supercluster classification. Ks° galaxy counts show an enhancement of ∼1.2 over the survey area for galaxies brighter than MK* at the VSCL distance, and a galaxy overdensity of δ = 0.50–0.77 within a photometric redshift shell around the VSCL, when compared with various Two Micron All-Sky Survey samples. Taking account of selection effects, the VSCL is estimated to contribute vLG ≳ 50 km s−1 to the motion of the Local Group [ABSTRACT FROM AUTHOR]