Rumours about a relationship between the two young actors began early last month, with sources saying that the couple met through a mutual friend in January.
A source told the Daily Mail: "Amanda's been flying home to Los Angeles when she's not in the studio. She has been trying to see Josh but it hasn't worked. She decided to end things."
Seyfried split from longtime boyfriend Dominic Cooper in May 2010, which she said in an interview still hurts despite the fact that the two remain friends.
The 26-year-old actress recently wrapped Lovelace, in which she plays the troubled porn star Linda Lovelace.
She is currently filming alongside Anne Hathaway and Hugh Jackman in an adaptation of Les Miserables.
She most recently starred in the mystery thriller Gone.
Watch Amanda Seyfried in a trailer for Gone below: