The actress, 22, is rumoured to be dating DJ Sam Ronson, but apparently began texting Phelps after his eight gold medal victories in Beijing.
According to The Mirror, Phelps has shown friends messages on his phone, which say: "It's Lindsay. It's Lindsay."
"At first [Phelps] thought it was a joke when she got in touch," said a source. "But she's since been phoning and texting constantly.
"It's become a running joke in his camp. Lindsay has been trying to meet up with him. They're both going to the MTV Video Music Awards next month so they'll be sure to see each other properly then.
"Phelps is certainly flattered by all the attention but it's not clear how seriously he's taking it all."