The pair were spotted partying the night away at Belvedere's party at Le Baron in Cannes on May 18.
Witnesses said they were "very close" and spent the whole night laughing, joking and looking into each others eyes.
Cooper, the star of Mamma Mia, and Seyfried - who is currently in the box offices in her movie "Letters to Juliet" split up a few weeks ago.
Lohan was in for the kill.
A source said to HollywoodLife.com: 'Lindsay and Dominic were flirty, but not overly touchy. They arrived in a group and went to their table.
'Both were in great moods, dancing, having fun. They were really playful with each other.'
The source added: 'They were mainly hanging out with each other, though. They left together right before Grace Jones' performance ended.'