Johnson's rep denied pregnancy rumors Monday to People after the 29-year-old actress attended a party festooned with pink and blue balloons.
Johnson's rep said the bash was in honor of the Fifty Shades star's birthday, which fell Oct. 4. The party took place Sunday at Martin's house.
TMZ had reported Johnson's pregnancy after photos showed pink and blue balloons floating over Martin's house. Johnson's parents, Melanie Griffith and Don Johnson, Julia Roberts and Sean Penn were among the guests at the event.
Johnson and Martin were first linked in October 2017. Griffith told reporters in May she approves of Johnson dating the 41-year-old Coldplay singer.
"I adore him!" the actress said. "But she is very private about her life and I respect that."
Johnson confirmed her relationship with Martin in the November issue of Tatler.
"I'm not going to talk about it, but I am very happy," she said.