Sharing the message via Instagram, Sean also posted a screenshot from their first meeting on The Bachelor. In the snap, Catherine is feeding her future husband ice cream.
Lowe, 33, recalled that meeting in his love note.
"Happy birthday to the cute, bubbly, slightly awkward girl who fed me ice cream the first night I met her. I knew there was something special about you when after introducing yourself, you asked me to save you a dance inside at the Bachelor Mansion. It made no sense considering there was no dancing or music going on but your nonsensical comment intrigued me and I wanted that dance," he wrote.
Lowe's message continues: "I'm so incredibly thankful that I somehow convinced you to spend the rest of your life with me. You're the total package -- wonderful wife, fantastic mom and excellent jeopardy player. I love you weird ice cream girl."
ET's Lauren Zima caught up with the couple exclusively last month at their Dallas, Texas home, where they dished on why they're the only Bachelor couple to get married and stay married.
"Part of our success comes from the fact that we're not the typical Bachelor couple," Lowe said at the time. "We don't do a lot of the Bachelor reunions and go and meet up -- that's just not us. We live our own life and do our own thing, so we don't really feel that pressure and that's the way we like it."
See what else the happy couple had to say in the video below.