In a statement, the coastguard said: “Large boulders have been falling from the top of the cliff and have been rolling down onto the beach and into the water.”
It warned people not to climb or walk near the base of the clips and to keep dogs on leads.
The fall follows a landslip at nearby West Bay on 30 December that was captured on video.
The 95-mile-long Jurassic Coast is a Unesco World Heritage Site recognised for its outstanding rocks, fossils, and landforms.
There has long been concerns about land stability around Monmouth Beach – nine chalets overlooking the beach were demolished following a landslip in 2013.