According to the Daily Mail, the American actress, 24, and the Canadian actor, 24, were in Canada so that Miller could meet the parents of her new love.
Miller, who starred in Alfie and Casanova, and Christensen, who starred in Star Wars Episode II and III, met while filming Factory Girl together.
A friend of the actress told the paper, "Hayden was a brilliant shoulder to cry on when she finally split from Jude Law."
She added, "Sienna is quite flirty and their relationship became something more. Hayden is besotted with Sienna but I’m not sure she totally reciprocates his feelings."
"Ever since they stopped filming Factory Girl he has been calling and texting her – he thinks he’s in love."