Khan made the announcement as part of London Tree Week, a series of fun events that highlight the capital's green spaces.
The mayor joined members of the Woodland Trust for a preview of a giant 3D tree which was displayed on 31 May outside City Hall. The 'Wandering Tree' is a piece of street art creating an optical illusion. Members of the public can take pictures of themselves standing atop the "oak".
Khan said: "It is vitally important we protect our city's precious green spaces and that must include opposing building on the Green Belt. I've now met with my team of planners to make absolutely clear that this must stay at the forefront of planning decisions.
"Ensuring everyone in our city, especially young people, has access to green spaces from parks and playing fields to community gardens is vitally important. London Tree Week is a great reminder for us all to make better use of and enjoy our green spaces and I hope as many people as possible can enjoy some of the events during Tree Week. "
London Tree Week, which runs until 5 June, will see many of the capital's parks offering a variety of green trails and activities. They range from uncovering the history of Crystal Palace Park, to discovering the mulberry tree heritage around the City of London, and lessons in how to care for trees, held at Beddington Park.
Over the bank holiday weekend Londoners also had the chance to climb up an oak tree in Highgate Wood using CanopyStair, a spiral staircase, letting them view the woods from a new angle.