The 46-year-old Cruel Intentions star said she was diagnosed in August but has had symptoms for years.
“I am disabled. I fall sometimes. I drop things,” she wrote, calling the news “overwhelming in the beginning”.
Multiple sclerosis is an incurable condition affecting the brain or spinal cord, causing vision, balance and muscle problems.
According to the UK’s NHS, it is two to three times more likely in women than men, and is usually diagnosed in people in their 20s and 30s.
The cause of the condition is still unknown.
Blair is set to appear in the Netflix programme Another Life, and said the “profound” help she received from costume designer Allisa Swanson had inspired her to come forward with the story.
In her post she said she was “in the thick of it” but hoped to give hope to others by talking about the condition.