A man has been arrested on suspicion of vandalising a painting by the artist Mark Rothko, police said.
The 26-year-old was arrested by Sussex Police - on behalf of the Metropolitan Police - at an address in Worthing.
The valuable work was defaced on Sunday at London's Tate Modern art gallery.
The suspect was arrested on suspicion of causing criminal damage and is in custody at a Sussex police station.
The suspect was named as Vladimir Umanets - a founder of Yellowism, which he describes as "neither art, nor anti-art".
Mr Umanets, who studied art, said he was not a vandal.
The Tate Modern was shut for a short period on Sunday and then reopened after the incident at 3.25pm.
The gallery said it does not have a price for the defaced piece, but paintings by the Russian-born artist often fetch tens of millions of pounds.
A spokeswoman said: "There was an incident at Tate Modern in which a visitor defaced one of Rothko's Seagram murals by applying a small area of black paint with a brush to the painting.
"The police are currently investigating the incident."
Eyewitness Tim Wright (@WrightTG) posted on Twitter: "This guy calmly walked up, took out a marker pen and tagged it. Surreal.
"We gave a description to the gallery. Very bizarre, he sat there for a while then just went for it and made a quick exit."
A picture he uploaded to the social networking website showed five or six words scrawled on the bottom-right corner of the piece. Black streaks of paint ran down from the daubed writing.
Earlier this year, Rothko's Orange, Red, Yellow was sold for £53.8m - the highest price ever paid for a piece of post-war art at auction.
The 1961 painting went under the hammer at Christie's in New York.