The second season of the American television series Supergirl, which is based on the DC Comics character Supergirl / Kara Zor-El, a costumed superheroine who is the cousin to Superman and one of the last surviving Kryptonians.

The season was ordered in May 2016, and unlike the first season which aired on CBS, this season was picked up by The CW. While the first season was filmed in Los Angeles, beginning with this season filming relocated to Vancouver, to reduce the high production costs of the series. Other exterior shots were taken in Frankfurt (Germany) and show the local Deutsche Bank Building, which stands as the new headquarter for the DEO. The season was filmed from July 2016 to April 2017. Alongside Melissa Benoist, who stars in the title role, principal cast members Mehcad Brooks, Chyler Leigh, Jeremy Jordan, and David Harewood return from the first season, and are joined by Chris Wood, Floriana Lima, and Katie McGrath. Former series regular Calista Flockhart returns in a recurring capacity.

The season premiered on The CW on October 10, 2016, and ran until May 22, 2017, over 22 episodes. It received overall critical acclaim, being viewed as an improvement over the first season. The series was renewed for a third season on January 8, 2017. This is the first season to air on The CW.

On May 12, 2016, Warner Bros. Television announced that Supergirl had been renewed for a second season of 22 episodes and would move to The CW from CBS. Ali Adler and Andrew Kreisberg served as the season's showrunners. It was also announced that, beginning with this season, production would relocate to Vancouver from Los Angeles.

In the second season, Kara Danvers / Supergirl and James Olsen end their romantic relationship, in favor of maintaining a platonic one. Kreisberg said regarding this decision, "We realized that the best scenes between [Kara and James] were just the nice, sweet scenes where they were being friends." He also said that while the theme of the first season was "how does Kara become Supergirl?", the theme of the second would be "how does Supergirl become Kara?". Kriesberg said another theme of the season was "about coming into one's own and becoming who you are", such as Winn Schott becoming who he is by joining the Department of Extranormal Operations (DEO), and J'onn J'onzz embracing his Martian Manhunter persona, which he spent 300 years concealing, but does not have to any longer. The season is split in two-halves each with their own self-contained "big bad" story: Project Cadmus serves as the "big bad" of the first half, and Queen Rhea for the second half.

The season establishes that Alex Danvers is gay, with Maggie Sawyer being her partner. Chyler Leigh, who plays Alex, explained, "it's not like this thing that all of a sudden is just spilling out, it's a discovery for her". She added, "You have so many stories [...] where people are already established as gay, lesbian, bi[sexual]; these are people who are coming in like that" and described the second season as a "great opportunity to show somebody who's figuring it out, the light bulb moment and putting the puzzle pieces together." While the first season showed that the DEO operates in an isolated cave located "in the middle of nowhere", the second season shows that the DEO has relocated to their other base, a skyscraper in National City. Kreisberg explained that the creative team of Supergirl "fell out of love" with the DEO's cave setting, saying "It was cool in the beginning, and it just for some reason didn't feel like it was bringing everything together", hence the reason for shifting to the skyscraper setting.

Melissa Benoist, Mehcad Brooks, Chyler Leigh, Jeremy Jordan, and David Harewood return from the first season as Kara Danvers / Supergirl, James Olsen, Alex Danvers, Winn Schott and J'onn J'onzz / Martian Manhunter, respectively. They are joined by Chris Wood and Floriana Lima, portraying Mon-El and Maggie Sawyer, respectively. Harewood also portrays Hank Henshaw, who becomes this series' version of Cyborg Superman. The season establishes Olsen as the vigilante Guardian, unlike the comics where Jim Harper is the vigilante. The producers decided to depict Sawyer as a Latino for the series, unlike the comics where she is a white blonde; however, the character's homosexuality was retained for the series.

With the move of production to Vancouver, it was unclear if Calista Flockhart, who played Cat Grant in the first season, would remain with the series, as her original contract stipulated that she work near her home in Los Angeles. The CW president Mark Pedowitz said Flockhart wanted to remain with the series and that "We're in ongoing discussions... we're happy to have her in [in whatever capacity] works out." Flockhart ultimately reached a deal to be recurring in the second season. Andrea Brooks originally auditioned for the role of Sawyer, although she herself admitted that she did not feel that role was a "good fit". She was subsequently sent a further script to audition for the role of Janice. Having won the role, it was not until she received her first episode script that she discovered she would in fact be playing the recurring role of Eve Teschmacher. The character's name is a reference to a character of the same name played by Valerie Perrine in Superman (1978) and its 1980 sequel. Katie McGrath also joined this season in the recurring role of Lena Luthor.