"God put us together and continues to bless our marriage," she added.
"I couldn't imagine this life without you, @mfisher1212 I am thankful for these past six years together (and, of course, for our beautiful son, Isaiah) and I look forward to spending the rest of our lives in love."
Underwood and Fisher have been on vacation since mid-last week, reportedly in Cabo San Lucas, Mexico.
Fisher followed his wife's post by sharing a throwback picture from their wedding day.
"6 years ago today we were married. Time sure does fly when you're having fun and you marry up. I'm very grateful to have an amazing wife and mother by my side!" he wrote.
Underwood and Fisher wed at the Ritz-Carlton Reynolds Plantation in Greensboro, Ga., in 2010, and welcomed son Isaiah in February 2015.
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