The 48-year-old actor marked the occasion Tuesday by voicing his love for Harper and Gideon, his daughter and son with his husband, David Burtka, on Instagram.
Harris shared a photo of Harper and Gideon posing together on a boat.
"Happy 11th Birthday, Harper and Gideon. You continue to illuminate our world with your spirits, and are growing into truly remarkable people," Harris captioned the post.
"Thank you for pushing yourselves, for protecting each other, and for making your father and I laugh, melt, and marvel. You're the best things in our very fortunate lives. We love you both in every way, and without conditions," he said.
Burtka also posted photos of Harper and Gideon on Instagram.
"Happy Birthday to the most amazing kids I know. Thank you for bringing so much joy into our lives. We are so lucky to have you in our lives! I can't believe you are 11," he wrote.
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