Walkers have been warned to keep away from the site at Seaford Head, with the coastguard warning it could fall away at any time.
Amanda Death, who lives in Seaford, spotted the crack and alerted Newhaven Coastguard after going out for a walk with her daughter.
Coastguard spokesman Graham Easton said there had been cliff falls at Seaford, Birling Gap and Hastings this year.
"It could go at any time but it could hang on for quite a long time," he said.
"The advice is simple to stay away from cliff edges - but equally we want people to stay away from the base of cliffs.
"They can be crumbly and fall away and obviously a big crack like this would cause a massive cliff fall but even small rocks will cause a lot of damage if you're underneath at that sort of height."
BBC reporter Lucinda Adam estimated the crack was about 6ft (1.8m) to 7ft (2.1m) long.
She said the land above the crack was on the other side of the safety fence, closest to the sea.
Mrs Death said she went to have breakfast on the beach with her daughter on Saturday and spotted the crack when the pair walked further along the coast.
She said she stayed back from the edge but was able to zoom in on the crack with her phone and added: "It looks like it could go pretty soon."
Seaford is the start of a six-mile (10km) stretch of protected limestone sea cliffs that lead up to Beachy Head, according to Natural England.
Walkers follow a cliff top footpath from Seaford to reach Hope Gap and Cuckmere Haven.
A large crack has appeared in a cliff face in East Sussex.