The 20-year-old English actress shared a first picture of the 27-year-old American singer on Instagram after sparking dating rumors with Jonas in the fall.
"Miami Daze," she captioned a snapshot of the DNCE frontman smoking a cigar on a boat.
Turner and Jonas celebrated Halloween together in October 2016 and were spotted kissing at a Kings of Leon concert the next month. Sources told People in December that the pair are "dating exclusively."
"[Joe] was playing the field, but he really likes her and is willing to settle down with her," an insider said.
Turner was previously linked to The Vamps singer James McVey, while Jonas dated model Gigi Hadid. Jonas told People in July that he values a girl who gets along with his friends and has a sense of humor.
"The biggest test is bringing her around friends because I have friends who are really tough about new people -- we have a really tight circle," the star revealed.
"I would say she's gotta have a sense of humor and find my weird sense of humor funny," he added.
Turner is best known for playing Sansa Stark on Game of Thrones, which will return for an abbreviated seventh season in the summer. Jonas came to fame with brothers Kevin and Nick Jonas as the Jonas Brothers, and formed DNCE in 2015.