Most previous studies on marine hydrokinetic energy harvesters focussed on clean cylindrical oscillators. The current paper presents the results of the first investigation on the vortex-induced vibrations (VIV) hydrokinetic energy conversion from cylindrical oscillators covered with soft marine fouling. The one-degree-of-freedom system oscillates in the cross-flow direction. The Reynolds number ranges from 5.5 × 103 to 5.5 × 104. A hydroid-type of soft marine fouling is modelled with three different compaction ratios. On average, under a variety of electromechanical coupling and flow velocities examined, the marine fouling reduces the power harnessed and the energy transfer ratio by around 59 % and 62 %, respectively, relative to the clean oscillator.