The former Preston striker stole in at the near post to steer home Liam Lawrence's mishit shot midway through the second half after a spell of extended pressure.
Portsmouth's leveller was just reward for an impressive second half having previously seen a Nugent goal ruled offside, with Dave Kitson also firing another chance agonisingly wide and Hayden Mullins hitting the upright.
Steve Cotterill's side, however, looked second best throughout the majority of the first half and deservedly went behind to a Liam Trotter header.
Nugent, though, thwarted the Lions' hopes of securing their second victory at Fratton Park in 18 attempts and, in truth, were lucky to escape with a point.
Source: PA
Source: PA