Bright blue crayon markings were scrawled across the face, arms and torso of the 230-year-old Sabrina statue at Croome, Worcester.
A memorial to landscape architect Capability Brown was also defaced sometime during 8 April, the National Trust said.
The markings have been removed from the statue but work to clean the memorial is ongoing.
Carved from Coade stone, the sculpture is thought to have been made in 1802 and depicts the Roman Naiade, Sabrina, in a grotto which was originally decorated with shells, coral and gems.
We are dismayed that this has happened,” said a National Trust spokesperson.
They added: “Disappointing as they are, incidents like this are very rare considering the millions of visitors who enjoy and respect the places in our care.”
Work to remove crayon marks from the Lancelot ‘Capability’ Brown Memorial are ongoing. His work landscaping the grounds is thought to have been his first large commission.