The pair spent the day with their 3-year-old son Axl Jack at the beach, professing their love for each other on the social media platform.
"8 years!! Love you babe," Duhamel captioned a snap of the family giving the camera their best "hang 10" hand signs.
He also shared another photo with the same caption, posting a snap of Fergie and Axl playing in the sand.
Fergie later posted her own photo from the day, sharing a sweet snap of herself and Duhamel with the caption, "happy 8th anniversary to my bae."
The singer and Duhamel married in 2009, and welcomed Axl in 2013. They recently celebrated the New Year's holiday in Kihei, Hawaii, People shares.