The 32-year-old singer said that she first experienced the condition as a teenager, reported Digital Spy online.
"I have had to deal with depression," Williams said.
The singer said that she has never needed medication to treat her depression but it has been tough to overcome her issues.
"I had to choose to get out of bed and do whatever I needed to do to be happy," she added.
The singer also encouraged those suffering from the condition to seek help and not feel embarrassed to talk about mental illness.
"Go see a professional so that they can assess you. It's okay if you're going through something. Depression is not okay, but it is okay to go get help."
Williams has recently reformed with Destiny's Child for the release of a new greatest hits compilation, Love Songs. The album includes one new song, 'Nuclear', which was produced by Pharrell Williams.
It is the first new material released by the group since 2004 and it is rumoured that Williams and Kelly Rowland will join Beyonce Knowles on stage at her half-time Super Bowl performance next month.