Traffic in both directions is being diverted to the M4 Second Severn Crossing.
The closure, until 22:00 BST on Sunday, is to allow for routine maintenance work on the bridge.
The Grade I-listed bridge, which orig­i­nally car­ried the M4 until the Sec­ond Sev­ern Cross­ing opened in 1996, carries about 17,000 vehicles per day.
The M48 Severn Bridge is closed all weekend for maintenance work.