Dianna Elise Agron was born on April 30, 1986, in Savannah, Georgia, to Mary (née Barnes), a seamstress, and Ronald S. "Ron" Agron, a general manager of Hyatt hotels. Her father was born to a Jewish family, while her mother converted to Judaism before they married. Agron has a younger brother, Jason Agron, a photographer. She is Ashkenazi Jewish, of Russian-Jewish descent; her father's family were Jewish immigrants from Novgorod-Seversky in Ukraine, and the family's original surname was Agronsky. She is distantly related to Gershon Agron and Martin Agronsky. Agron was raised in San Antonio, Texas, and Burlingame, California; her family lived in various hotels due to her father's career, but her mother made sure that Agron and her brother knew this was not the norm. Agron has said that there was always music from the 1960s and 1970s playing at their home, and that her mother sheltered her (though not her brother) from watching contemporary films and television even as a teenager, opting to let her watch mostly classic musicals because she felt they had "a certain amount of loveliness to [them]". Being exposed to the "fairytale" and "fantastical" image of Hollywood from these films influenced Agron's decision to pursue acting, while her interest in storytelling comes from seeing different lives unfold around her growing up in the "fishbowl environment" of hotels.
Agron lived in Texas from the age of two until she was nine, and took up dancing at the age of three, studying jazz and ballet, and later hip-hop dancing. She often performed in local and school musical theater productions, including as Dorothy Gale in The Wizard of Oz when she was eight. She attended Hebrew school growing up and was educated at a Jewish day school through third grade. She was bullied harshly for her Jewish faith while living in Texas and noted that she assumed having police guarding their Temple was normal until the family moved to California, adding that being Jewish was a large part of her identity as a child because of how it ostracized her. When the family moved to California, Agron attended Lincoln Elementary School, Burlingame Intermediate School, and Burlingame High School in the Bay Area; she attended religious school and had her bat mitzvah at the Reform Judaism-practicing Peninsula Temple Sholom. She said that she found it much easier to make friends there than she had in Texas, though she described her middle school experience as sometimes unpleasant, giving the example of a boy following her around and calling her a man when, aged thirteen, her voice dropped significantly. This gave her a complex about how her voice sounded and she avoided speaking and singing in her natural register for a long time, though she overcame this and credits it with giving her a thick skin.
In high school, Agron was on the homecoming court in both her junior and senior years, tying for homecoming queen with a friend; she has said she was not "popular" in a stereotypical sense in high school, but had many friends in different groups. She was involved in school theater, performing in Vanities and Grease as a senior, and helping with set design, costumes, and painting. Agron has broken her nose twice. The first break occurred when she was fourteen, but she did not have it repaired until it was damaged again on a day off during the Glee tour. She was also injured in a traffic collision and underwent physical therapy in high school. As a teenager, Agron was a dance teacher and worked at a local boutique, Morning Glory, where she "became enthralled with fashion". Though her mother dressed her in doll-style dresses, she began experimenting with fashion in high school. She took piano lessons and said that she came to love photography in high school, as well, where she learnt on film. When she was a teenager, her father became ill with the stroke disorder CADASIL. The family thought it was multiple sclerosis after tests proved indefinitive; Agron was not made aware of his illness until she was fifteen, when he had a stroke and began losing his cognitive and physical abilities. She spoke to Cosmopolitan about the impact of this on her family, which caused her parents' marriage to fall apart, saying the separation was devastating for her and her brother. She added that she "had to play therapist to [her] family[,] be the glue". She later said that, when her father became ill, he "lost his faith for some time" and the family stopped attending Temple.
