Summer Roberts is a fictional character on the FOX television series The O.C., portrayed by Rachel Bilson.

Summer was originally intended as a small supporting character, only there as an object of fantasy for Seth Cohen (Adam Brody) and a friend for Marissa Cooper (Mischa Barton), while Ryan Atwood (Ben McKenzie) and Marissa were the lead couple. However, due to Bilson's performance, Summer became an increasingly important character. Series creator Josh Schwartz said: "She was adorable, funny and smart, which was a really fresh way to go with that character. And the more we gave her to do in the first couple of episodes, the more she kept nailing. Finally, in the episode with the Tijuana road trip, we watched a scene between her and Seth in the diner and knew: This girl is amazing. Let's make her a series regular."

Summer Roberts grew up in Newport Beach, California, with her wealthy father, Dr. Neil Roberts (Michael Nouri). For much of the series, she also lived with a stepmother, named Gloria (unseen character), whom she never refers to by name, instead calling the latter "what's her name" or "the stepmonster" due to her mood swings and various addictive pills. She was often paired in scenes with her best friend, Marissa Cooper (Mischa Barton), who was the leading female of the series until her death. Summer became the female lead character of the show's fourth season. During this all, she had a long lasting relationship with Seth Cohen. In a flash forward in the series finale, "The End's Not Near, It's Here", she marries Seth. Her character was only scheduled to appear in a few episodes, but was considered a hit, and ended up being in each of the series. Spoiled by her divorced father, Summer considers him to be her best friend, and his opinion matters as much, if not more than her own.

Summer was unusual in combining the qualities of a conventionally attractive and desirable character with a tough, outspoken, "take-charge" attitude. She is portrayed as highly sexualised and initially appears unintelligent; however it is revealed that she has maintained strong academic performance throughout high school when she is accepted into Brown University, where she nurtures a passion for environmental activism.

In season 4 Summer adopts a pet rabbit and names her Pancakes. It is revealed in the season finale that Pancakes had been pregnant, after Seth and Summer discover Pancakes' babies living under her bed. Ryan adopted one of the bunnies, naming it Flapjacks.

In the first season, Summer is introduced as a high schooler who is obnoxious and snotty. Her elitism is on display in the first episode, where she is at first attracted to newcomer Ryan Atwood – until she learns he is from Chino, prompting her response, "Chino, ew." Her attraction to him continues afterwards. However, different sides to her character have been shown as she has developed. Before choosing Summer (after her finally showing her feelings for Seth) Seth picks Anna Stern (Samaire Armstrong), who exhibits a similarly strong personality and shares his sarcastic sense of humor and love of comic books, the first go around. For the few episodes, they have a "will they or won't they" type of relationship, but they eventually begin dating and fall in love. In the process of their characters' relationship, Summer and Seth experience many struggles because Summer thinks it will affect her status quo at their high school.

As the season progresses, Summer is affected by their relationship and is often jealous of Seth, since he seems to be enjoying Anna's company so much. As a result, Summer gets angry and starts dating a classmate named Danny (Bret Harrison), but that relationship doesn't last long, and she finds that she still has feelings for Seth. Midway through the end of the first season, Anna breaks up with Seth because it is obvious he is still in love with Summer. Seth and Summer become a couple, and announce their romance at a school kissing booth, after which they become very close. Summer and Seth lose their virginity to each other. When Ryan moves away, however, Seth, missing his adopted brother, moves away as well at the end of the season, leaving behind a heartbroken Summer.

Summer has a new boyfriend, Zach Stevens (Michael Cassidy). Zach is starting to become friends with Seth, whom he met shortly before Zach and Summer started dating, yet their friendship is a little mixed since they both have feelings for Summer. Summer seems committed to putting Seth behind her, although at times it becomes clear that she still has feelings for him. This inevitably causes problems for her relationship with Zach, who suspected they still held feelings for each other. This is further complicated when Summer also becomes professionally involved with Seth and Zach as they develop their comic book, Atomic County, for publication. This relationship proves inextricably linked with the personal feelings all have for each other, and resulted in a bad meeting with a media executive that effectively killed their partnership.