The actor took to Instagram to share an emotional tribute to Kate, who was married to David's brother Andy, hours after she was found dead.
"Fuzzy picture but i love it. Kate and I during Christmas family photos. We had so much fun that day," the 53-year-old wrote.
"She was so sharp and quick on her feet. She could make me laugh so hard. I still cant believe it. Its a rough world out there people, try to hang on."
The Kate Spade New York designer was found dead on Tuesday of an apparent suicide.
In a statement to the New York Daily News, her family said they are “devastated by today’s tragedy.”
“We loved Kate dearly and will miss her terribly. We would ask that our privacy be respected as we grieve during this very difficult time.”