The Kouda sisters watched their father leave them to start a life with his mistress 15 years ago. Soon after their mother abandoned them, forcing their maternal grandmother in Kamakura to raise them. Now a working adult, Yoshino Kouda receives a phone call with news from her older sister Sachi: their father has passed away from cancer. Although Yoshino is not distraught by the news, she—along with her younger sister Chika—feels obliged to go to the funeral, as Sachi is unable to due to her work schedule.

At the ceremony, the pair meet Suzu Asano—their 13-year-old half-sister. After Suzu's own mother's death, their father remarried. Suzu became the one to shoulder every responsibility, including taking care of her father's deteriorating health. Despite Suzu being a living reminder of their father's infidelity, Yoshino and Chika cannot help but feel sorry for her.

Surprisingly, Sachi arrives at the burial as well, witnessing the struggles Suzu has gone through. A newfound sympathy makes Sachi decide to bring Suzu into their immediate family. The four women must now learn how to navigate through life and come together as sisters.

Umimachi Diary won the 11th Japan Media Arts Festival's Excellence Award in 2007 and the 61st Shogakukan Manga Award in the General Category in 2015. In 2008, it was nominated for the 1st Manga Taisho Award, placing third; it later won the 6th Manga Taisho Award in 2013. The series was also nominated for the 12th Tezuka Osamu Cultural Prize in 2008, the 13th Tezuka Osamu Cultural Prize in 2009, and the 23rd Tezuka Osamu Cultural Prize in 2019. The manga ranked 10th on Da Vinci magazine's "Book of the Year" list in 2019.

The series was published in French by Kana from April 18, 2013 to August 30, 2019, in Italian from April 12, 2017 to August 7, 2019, and in Spanish by Panini Comics.

Umimachi Diary was adapted into a live action film, which premiered on May 14, 2015 at the Cannes Film Festival.