Mrs Smith and Suzanne Main announced they planned to register a civil partnership three years after revealing their relationship. The former city councillor became the first openly lesbian MSP when she went public in May 2003.
Mrs Smith, who has two sons, was divorced in August 2003. Mrs Main, the mother of three children, left her husband after starting the relationship with Mrs Smith.
A statement said: "Margaret Smith MSP and Suzanne Main are happy to announce their intention to register a civil partnership next month.
"The ceremony will take place in Edinburgh in the presence of family and friends."