Keith Gregory, 63, was arrested on Saturday after allegedly damaging Thomas Gainsborough's painting "The Morning Walk" in the prestigious London museum.
The gallery said in a statement that the attacker had used a "sharp instrument".
"The damage is limited to two long scratches which have penetrated the paint layers but not the supporting canvas," the statement said.
"A man was quickly detained by gallery assistants with the help of some visitors. He was arrested by the Metropolitan Police," it added.
The east wing of the gallery, where the permanent exhibition of British paintings is housed, was evacuated and closed for two hours.
"The Morning Walk" depicts a young couple walking through a woodland landscape and was completed in 1785, shortly before the young lovers were married.
It could be seen in the background in a scene in the 2012 Bond film "Skyfall" starring Daniel Craig.
Gregory was charged on Sunday and remanded in custody.
The painting has been removed from the gallery to be assessed by restorers.