Seyfried starred alongside Channing Tatum in Dear John, the film adaptation of the novel of the same name that was written by Nicholas Sparks. The film was released on February 5, 2010, and received generally negative reviews. Seyfried wrote and recorded "Little House", a song on one of the soundtracks of Dear John. Kirk Honeycutt of The Hollywood Reporter said, "Seyfried gives the character and her relationship all she's got, but she can't do all the heavy lifting. The romance is too one-sided, and frankly, you can't blame her for steering her life into another channel." Despite the mixed reviews, Dear John became the first film to break up Avatar's box office reign at number one at the United States box office and grossed US$80 million in the US theatrically and US$115 million worldwide.
Seyfried appeared as the title character in the erotic thriller Chloe, theatrically released by Sony Pictures Classics on March 26, 2010. Chloe originally premiered at the Toronto Film Festival in September 2009. In the film, Seyfried's character is an escort who is hired to test a husband, because his wife feels that she cannot trust his fidelity. Chloe enjoyed commercial success and became director Atom Egoyan's largest financial success. Seyfried's performance in the film also received favorable reviews from critics; it also helped her to gain industry acclaim and receive more opportunities to play more varied roles.
Later in 2010, Seyfried starred in the romantic-comedy film Letters to Juliet, based on the book by Lise and Ceil Friedman, which was released to mixed reviews and was a box office success, generating US$80 million worldwide. She was named, and received an award as, the "Showest Breakthrough Female Star of The Year". She also won the "Scared-As-S**T" category for her performance in Jennifer's Body and was nominated for Best Female Performance for her movie Dear John, at the 2010 MTV Movie Awards. Also in 2010, she was included in Forbes's "The 17 Stars To Watch" list, and received three nominations in the Teen Choice Awards, including for Choice Movie Actress Drama and Choice Movie Chemistry with her co-star Channing Tatum for their film Dear John. Seyfried was also nominated for Choice Movie Actress Romantic Comedy for Letters to Juliet.