The 53-year-old has been at the St James' Park helm for the past decade since handing over £134m to purchase all of the club's shares.
In an interview earlier this year, however, Ashley suggested that new backers would be required if Newcastle were to challenge in the higher reaches of the Premier League.
A club statement read: "As one of the Premier League's oldest and best supported football clubs - and for the benefit of its many fans and supporters in the UK and across the world - Newcastle United requires a clear direction and a path to a bright and successful future.
"To give the club the best possible opportunity of securing the positioning and investment necessary to take it to the next level, at what is an important time in its history, its present ownership has determined that it is in the best interests of Newcastle United and its fans for the club to be put up for sale.
"To give an incoming owner the maximum possible flexibility to make meaningful investment in the club, including in its playing squad, the sale process will give interested parties the opportunity of deferring substantial payments.
"A process will now commence of identifying new ownership for the club that will be capable of delivering the sustained investment in and dedication to the club that is necessary for it to achieve its ambitions."
Ashley's lawyer Andrew Henderson went on to confirm to Sky Sports News that a sale will ideally be completed in the next 70 days.
Reports earlier this month claimed that a number of unnamed potential investors have stepped forward in recent weeks, with non-disclosure agreements already being signed.