The duo were first romantically linked in 2008 but they later split.
However, the couple were spotted spending time together in New York this week, X17 Online claims.
A source also said that the pair were "whispering and laughing" and added that they "left the club holding hands".
"They didn't care who saw them, and they definitely seem to be back on," the insider alleged. "It was just like the old days - they couldn't get enough of each other!"
Last month, reports claimed that Lohan - who recently landed the role of Elizabeth Taylor in a television biopic - is planning to prioritise her career over her romantic life.