We report the discovery of NGTS-2b, an inflated hot-Jupiter transiting a bright F5V star (2MASS J14202949-3112074; $Teff$=$6478-89+94$ K), discovered as part of the Next Generation Transit Survey (NGTS). The planet is in a P=4.51 day orbit with mass $0.74-0.12+0.13$ M, radius $1.595-0.045+0.047$ R and density $0.226-0.038+0.040$ g cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-3</mrow>}$; therefore one of the lowest density exoplanets currently known. With a relatively deep 1.0% transit around a bright V=10.96 host star, NGTS-2b is a prime target for probing giant planet composition via atmospheric transmission spectroscopy. The rapid rotation ($vsin$i=$15.2\pm 0.8$ km s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) also makes this system an excellent candidate for Rossiter-McLaughlin follow-up observations, to measure the sky-projected stellar obliquity. NGTS-2b was confirmed without the need for follow-up photometry, due to the high precision of the NGTS photometry.