The 32-year-old Phillips, an Olympian, is the daughter of Princess Anne and Capt. Mark Phillips. The child will be Anne's third grandchild, as her son, Peter Phillips, has two daughters.
Zara, the 14th in line to the British throne, competes in three-day eventing, a grueling sport that combines the disciplines of dressage, show jumping and cross country. She competed in the 2012 London Olympics and won silver.
The palace made the announcement Monday, just before the expected birth of the heir to the throne. Prince William and his wife Kate are expecting their first child sometime this month.
Phillips and Tindall were married in July 2011 in Edinburgh. Their baby is expected in 2014.