"3.6.15 One year down!," the pop star wrote on Instagram alongside a photo of Harris' gift, a gold heart-shaped locket with their anniversary date engraved.
On Snapchat, Harris shared a video of the couple getting excited for a homemade cake frosted with the words "1 year" and two red hearts.
"One year, we got one-year cake!," the noted DJ said as Swift smiles and nods. The video was captioned with the headline, "1 year...that was easy."
Swift and Harris reportedly started dating after hitting it off at the 2015 Brit Awards but kept the relationship secret for months afterwards.
The couple last made headlines in February when Harris congratulated Swift on social media following her big win at the Grammys. "Congratulations to my beautiful girlfriend," he wrote alongside a photo of her acceptance speech for winning Album of the Year.