She wrote on Instagram: "As many of you know, Kanye has bipolar disorder.
"Anyone who has this or has a loved one in their life who does, knows how incredibly complicated and painful it is to understand."
He is a "brilliant but complicated person" whose "words sometimes do not align with his intentions", she said.
The rapper is one of America's biggest music stars, and is currently attempting to run for US president. But his first campaign rally and a number of recent Twitter messages have sparked confusion and concern.