This is The Japanese Mask by the multi-first-named Gustave-Claude-Etienne Courtois (1852-1923). Wikipedia's short entry on him begins by telling us that he "was born to an unwed mother who was devoted to him" -- I can't help speculating that she gave him all those first names to make up for the fact that she couldn't give him what she would have considered a respectable surname. Lots more of his paintings can be seen here, and many of them are rather conventional portraits. This one's a bit different and I think it's gorgeous and fascinating. I love the way she's looking at the artist -- what is she thinking about? and I guess the contrast between her face and the face of the mask above gives it a bit of an edge.
