Bruce Jenner
Bruce Jenner Reuters

Former Olympic star Bruce Jenner wrote a touching letter to his daughters Kendall and Kylie to mark Father's Day.

"I have learned so much about raising kids over the last 34 years," he wrote on, "You guys are my ninth and tenth children, and my fifth and sixth girls to raise. Every girl has been totally different, both in personality and lifestyle. And that's the thing I've learned to value most: be true to yourself, be your own person-and be caring to others, as well."

The 63-year-old also praised his wife, American socialite Kris Jenner.

"Another key to life is picking the right partner to share it with. I certainly have learned that is really difficult to find, because people change. Your mom and I have been together for 23 years, and our relationship has constantly changed and evolved. But we have been able to make it through those adjustments because of a deep love and caring for each other. We are still as committed as ever. Go out and find that person who makes you a better person, and vice versa," he added.

Jenner also shared words of wisdom with his daughters.

"I'm so proud of the two of you. I see you both have a caring and enthusiasm for life. But in the future, you will face obstacles. Don't let them scare you. Fear is part of the game. Trying things outside of your comfort zone is essential, because your ability to grow as a person is directly related to the amount of insecurity you can handle. So be strong and fearless on this journey we call life," he concluded.

Meanwhile, other celebrity fathers, including Ethan Hawke, Mario Lopez, Richie Sambora, and Aaron Sorkin, have also contributed to the website.