"Best birthday ever ever. I have a new favorite four letter word. #JustVoted @WhenWeAllVote," the actor said on Twitter alongside a photo of himself posing with Lively as the pair hold up mail-in ballots.
The couple smiles in the photo, surrounded by colorful balloons.
"Happy Birthday to 2018's Sexiest Voter Alive @vancityreynolds #justvoted #absenteeballot party!" Lively posted on Instagram alongside a similar photo.
Reynolds, born in 1976, turned 42 Tuesday.
Reynolds' friend Hugh Jackman also wished the Hollywood star a happy birthday alongside a photo of the pair hugging each other.
"Because I'm told that I AM THE NICEST GUY and you're NOT. @VancityReynolds ... I will let you hug me. Just this once. On your birthday," Jackman said.