The 49-year-old man crashed and came off his bike.
He sustained fatal injuries and died at the scene close to the Megget reservoir, near Selkirk.
A spokesman for Tour O the Borders said: “It is with the deepest regret that we announce the death of a participant at Tour O The Borders closed road cycling sportive (Sunday 1st September 2019).
“Next of kin have been informed. No further information is available at this point but we expect Police Scotland to issue a statement in due course.
“We extend our deepest sympathy to the family and friends of the rider at this tragic time.
A Police Scotland spokesman said: “Police in the Scottish Borders responded to a road near Megget reservoir at around 9.40am on Sunday.
“A 49-year-old male cyclist was involved in a collision during the Tour O The Borders event.
“As a result the man came off of his bike and sustained serious injuries.
“No other persons were involved. He was pronounced dead at the scene. Inquires are continuing.”