Her husband, Mike Tindall, who has captained the English rugby team, announced the birth during an episode of his podcast, "The Good, The Bad and The Rugby," published Wednesday.
The baby boy was born on the bathroom floor of their home in Gloucestershire, southwest England, around 6 p.m., after he "arrived very quickly."
"It was running to the gym, get a mat, get into the bathroom, get the mat on the floor, towels down, brace, brace, brace," said Tindall.
Zara Tindall is the daughter of Princess Anne, the Queen's second child, and a first cousin of Princes William and Harry. The arrival is the monarch's 10th great-grandchild and is 22nd in line to the throne.
The baby, named Lucas Philip, weighed 8 lbs 4 oz at birth, a spokesperson for the couple told CNN.
The couple's other two children, Mia Grace and Lena Elizabeth, were not at the house on Sunday for the new arrival but have seen him since and are "over the moon," Mike Tindall said.