The royal couple has sent a Union Jack onesie baby suit for the actress's son Tennessee James, who was born Sep 27, reports femalefirst.co.uk.
Witherspoon, 36, met the couple when they had visited Los Angeles. Prince Williams and Catherine were among the first few people to send the gift along with a handwritten note attached to it.
This is the actress's first child with husband Jim Toth.
She shares custody of daughter Ava, 12, and son Deacon, 8, with ex-husband Ryan Phillippe with whom she was married for seven years, before calling it quits in 2006.