The former Heroes actress has been photographed enjoying a close date with the boxer at Beach Taco Shack in Miami, Florida, reports TMZ.
The couple previously dated for 18 months before splitting in 2011 due to their long-distance relationship.
She soon began dating American footballer Scotty McKnight, before breaking up after a year-and-a-half together.
It has been claimed that Ukranian Klitschko is planning to make regular trips to see her on the set of her current TV series Nashville.
"Hayden's so happy," a source told Life & Style. "They're definitely in love again and tighter than ever.
"They have been travelling together. He's even planning to visit her while she films in Nashville."
At the end of their romance, the boxer said in a statement: "We have had a great time together.
"But more and more we realised how difficult it is to maintain a relationship when living on two continents. I cherish Hayden as a person and woman and I'm sure we will stay friends."
Panettiere told TMZ in January that she and Klitschko were "just good buddies".
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