The Liberal Democrat leader was said to be "absolutely delighted". Aged 44, he became an MP at 23 and was married in 2002. Mrs Kennedy is 34, and a Camelot Lotto executive.
The baby is due in April, and could be a potentially helpful contribution in the general election campaign expected around then.
A statement from the MP for Ross, Skye and Inverness West said: "We have made no secret of the fact that we wanted a family, and we are very happy to be able to make this announcement."
After succeeding Paddy Ashdown in 1999 Mr Kennedy has confounded critics by lifting the Lib Dems' tally of MPs from 46 to 55. Coincidentally, he registered another political gain yesterday when his old ally and mentor, Lord McNally, was elected unopposed as leader of the Lib Dems in the Lords.