Sullivan has denied any illegal conduct and will step down from his role to 'avoid disruption to the Club while he addresses the matter privately'.
In a statement published on their website, West Ham said that interim Chief Executive Officer Karim Virani will now lead the club's regular operations.
"West Ham United can confirm that Joint-Chair David Sullivan has stepped down from his position with immediate effect," the club said.
"Mr Sullivan has also resigned as a director of both WH Holding Limited and West Ham United Football Club, having been made aware of the impending publication of serious historic allegations. It is understood that none of the allegations relate to West Ham United or any of its operations.
"Through his own legal representatives, Mr Sullivan denies any illegal conduct and has decided to step down in order to avoid disruption to the Club while he addresses the matter privately.
"Interim Chief Executive Officer Karim Virani, reporting into the current Board of Directors, will continue to be responsible for leading the Club’s day-to-day operations. The Club will provide an update on the future structure of the Board of Directors in due course, but will make no further comment at this time."
Sullivan has been involved with West Ham since 2010 and recently oversaw the club's relegation to the Championship after finishing 18th in the Premier League.
Fans have protested over his ownership for some time now, with the most significant coming in the 2025/26 season.
While no allegations have yet been made public, the 77-year-old denied any illegal conduct in a statement to the media.
"I have recently become aware that factually incorrect and entirely false, decades-old allegations concerning my personal life are due to be broadcast and published.
"The false allegations levelled against me have been sensationalised by the media. After a lifetime spent building businesses in the adult industry, in which I have met thousands of women, it is sadly inevitable that a small number of improper conduct claims are being made against me. I categorically deny these claims."