Managers say that the fee will prevent closures and help pay for cleaning and maintenance.
There are 27 toilet blocks across the eight parks, which are currently free and cost The Royal Parks around £1.5m a year to clean and maintain.
The new 20p charge will be introduced between February and March 2015, although accessible toilets, most of which have baby changing facilities, and children’s toilets in playgrounds, will remain free. There will be no charging at toilets in park cafes as they are not included in these plans.
Head of park services Dennis Clarke said: "Government funding for The Royal Parks has been cut by more than a third in real terms since 2010/11, so we face some tough choices. The 20p cost is a relatively modest charge compared with other toilet facilities in London. This will not generate any profits for The Royal Parks, but it will allow us to keep toilets open and reduce the amount by which we subsidise them."
Vinci Facilities has been appointed following a competitive tendering process and will take over responsibility for cleaning and maintaining the toilets this autumn. The introduction of charges will be publicised to park visitors well in advance.
The Royal Parks has said that each of the parks will have static or mobile attendants during opening hours, so toilets will be visited, checked and cleaned regularly throughout each day but some will only have a coin operated entry system with change machines located nearby. Attendants will be contactable by mobile phone and will allow free entry to people with particular needs or medical conditions.
Fees will be suspended during high-profile events held in the Royal Parks, such as Remembrance Sunday and the London Marathon.
The Royal Parks needs to generate 75 per cent of its income by 2015/16.