Roberta Ann Kaplan (born 1966), also known as Robbie Kaplan, is an American lawyer focusing on commercial litigation and public interest matters. Kaplan successfully argued before the Supreme Court of the United States on behalf of LGBT rights activist Edith Windsor, in United States v. Windsor, a landmark decision that invalidated a section of the 1996 Defense of Marriage Act and required the federal government to recognize same-sex marriages. She was a partner at Paul, Weiss, Rifkind, Wharton & Garrison before starting her own firm in 2017. In 2018, she co-founded the Time's Up Legal Defense Fund.

A native of Cleveland, Ohio, Roberta Kaplan grew up in a Jewish household. She graduated from Hawken School in Gates Mills, Ohio, in 1984. LGBTQ scholar and activist Aaron Belkin was Kaplan's high school friend and prom date. She earned a B.A. in Russian history and literature from Harvard University in 1988. While in college she spent a semester abroad in Moscow and "discovered a passion for political activism when she became active in the movement to free Soviet Jewry". She received her J.D. from Columbia Law School in 1991.

Kaplan served as a law clerk for Mark L. Wolf of the U.S. District Court in Massachusetts. While clerking for Judith Kaye, of the New York Court of Appeals, she assisted Kaye with a number of academic articles. Kaplan's scholarly articles include "Proof versus Prejudice" (2013).

Kaplan joined the law firm Paul, Weiss, Rifkind, Wharton & Garrison LLP in 1992 and made partner in 1999. She has served on the board and as chair of the board of the Gay Men's Health Crisis, which created the Roberta Kaplan Legal Center to provide free legal services.

In July 2017 Kaplan left Paul Weiss to start her own practice Kaplan Hecker & Fink LLP, a law firm dedicated to commercial litigation and public interest matters. She departed her firm in June 2024.

In 2009, Kaplan agreed to represent Edith Windsor pro bono. Windsor's wife, Thea Spyer, had died two years after they wed in Canada, leaving Windsor her sole heir. Because their marriage was not recognized under existing U. S. federal law, Windsor received an estate tax bill of $363,053. Windsor went to gay rights advocates seeking redress, but could find no one to take her case. She was referred to Kaplan, who later recalled, "When I heard her story, it took me about five seconds, maybe less, to agree to represent her." Kaplan had been co-counsel on the unsuccessful bid for marriage equality in New York state in 2006.

On June 26, 2013, the U.S. Supreme Court issued a 5–4 decision declaring Section 3 of the Defense of Marriage Act to be unconstitutional. Subsequent to Windsor, the Supreme Court ruling in Obergefell v. Hodges (2015) struck down all remaining state and federal laws against same-sex marriage across the United States. Kaplan wrote about United States v. Windsor in the book Then Comes Marriage.

In 2017, Kaplan and co-counsel Karen Dunn filed a civil lawsuit on behalf of students, clergy members and local residents against 15 individual defendants and associated groups for damages following alleged injuries sustained at the 2017 Unite the Right rally in Charlottesville, Virginia. The lawsuit is based on the Ku Klux Klan Act and according to The New York Times, the defendants are "an array of neo-Nazis, white identitarians and old-line pro-Confederates."