The incident took place on Perran Sands Beach, by Perranporth.
Emergency services were alerted on Sunday at 1.35pm.
Police have launched an investigation, and are urging witnesses to come forward.
According to Cornwall Live, a search of the coastline will continue through the afternoon.
The woman is reported to have been seen diving under a wave, then not resurfacing.
A spokesperson for Devon and Cornwall Police said: “Description is a white female in her 40s or 50s, slim build with dark shoulder length hair, wearing a short black dress.
“She is believed to be local because she informed lifeguards she is a regular swimmer in these waters.
“Police are asking for any information which may assist with the identity or potentially the whereabouts of this female.
“One possible outcome is that she may have got out of the water unharmed and is unaware this has been reported.
“However, witnesses have described her diving under a wave and not coming back to the surface.”
Police are asking anyone with information to call 101, quoting log number 585 070821.
In a separate incident, a man and women were found dead in a Cornish fishing village on Friday.
Police discovered the couple in a property in Polperro, a renown Cornish beauty spot.
The bodies, believed to be of a husband and wife, were found inside a house on Langreek Road.
In a statement Devon and Cornwall Police confirmed they are not seeking anyone in relation to the incident.
They said: “Police and ambulance crews attended the scene where a woman in her 40s and a man in his 50s were confirmed dead.
“Their next of kins have been informed.
“Police are not currently seeking anyone else in relation to this incident.
“A cordon remains in place and residents can expect to see officers in the area into the weekend.”
Polperro, a large fishing village, is typically popular with tourists during the summer.
More to follow…
