The British actress formed such a close bond with her G.I. Joe: The Rise of Cobra co-star Rachel Nichols the pair chose to mark the romantic day by spending time alone - away from the male-dominated film set.
Sienna said: "We needed each other - it was a very testosterone-fuelled environment - we actually spent Valentine's Day together, in my bed!" Rachel added: "We all got on so well making the film - we spent all of our time together."
Rachel isn't the first female co-star Sienna has become firm friends with. The 27-year-old blonde got very close to Keira Knightley when the pair starred in The Edge of Love.
Sienna explained: "Now we swap books. I went to Keira's house for dinner and we talked about books and swapped some. We play cards and do newspaper crosswords together.
"She's learning the flute at the moment and I'm learning the piano. I mean we are really quite nerdy."