Beauty always has an element of strangeness. I do not mean a deliberate cold form of strangeness, for in that case it would be a monstrous thing that had jumped the rails of life. But I do mean that it always contains a certain degree of strangeness, of simple, unintended, unconscious strangeness, and that this form of strangeness is what gives it the right to be called beauty.

[Le Beau est toujours bizarre. Je ne veux pas dire qu’il soit volontairement, froidement bizarre, car dans ce cas il serait un monstre sorti des rails de la vie. Je dis qu’il contient toujours un peu de bizarrerie, de bizarrerie naive, non voulue, inconsciente, et que c’est cette bizarrerie qui le fait être particulièrement le Beau.]

Charles Baudelaire (1821-1867) French poet, essayist, art critic
“The Universal Exhibition of 1855 [Exposition Universelle de 1855],” sec. 1 (1855) [tr. Charvet (1972)]
    (Source)

Frequently paraphrased as "Strangeness is a necessary ingredient in beauty." See also Bacon.

Collected in Curiosités Esthétiques, ch. 4 (1868). (Source (French)). Alternate translations:

The Beautiful is always strange. I do not mean that it is coldly, deliberately strange, for in that case it would be a monstrosity that had jumped the rails of life. I mean that it always contains a touch of strangeness, of simple, unpremeditated and unconscious strangeness, and that it is in this touch of strangeness that gives it its particular quality as Beauty.
[tr. Mayne (1965)]

Beauty is always bizarre. I do not mean to say that it is deliberately, coldly bizarre, for in that case it would be a monster that has escaped from the confines of existence. I mean that it always contains a certain amount of strangeness, naïve strangeness, unforced and even unconscious, and that it is this strangeness that stamps it as Beautiful.
[tr. Gregory (1961)]