Walking with a limp and guided by her friend, Rep. Debbie Wasserman Schultz, D-Fla., Giffords worked her way to the well, surrounded by other members of the Arizona delegation as Rep. Jeff Flake, R-Ariz., held her hand, MSNBC.com reported.
"I am so proud of my friend," a crying Wasserman Schultz said of Giffords. "It will always be one of the great treasures of my life to have met Gabby Giffords and to have served with her in this body."
While they won't see each other daily, "we will be friends for life," Wasserman Schultz said before reading Giffords' resignation letter.
"Even as I have worked to regain my speech, thank you for your faith in my ability to be your voice," Giffords said in her letter.
She also said she would focus on her recovery and return some day to public service.
"Every day I am working hard. I will recover and will return, and we will work together again, for Arizona and for all Americans," she said.
Giffords, aided by Wasserman Schultz, walked to the speaker's chair to deliver the letter, touching off a standing ovation.
Watching from the gallery were Giffords' husband, retired astronaut and Navy Capt. Mark Kelly, and her mother.
Before resigning, Giffords voted in favor of a bill she introduced shortly before being shot last January. The bill, which would create new penalties on smugglers who use small, low-flying aircraft to bring drugs into the United States, passed unanimously, 408-0.
Giffords was shot in the head during a meet-and-greet event in January 2011 outside a Tucson grocery store. Six people were killed and 13 people, including Giffords, were injured.
Thunderous applause and a standing ovation greeted Giffords Tuesday as she entered the chamber to listen to President Obama's State of the Union address. She walked to her seat holding hands with fellow Arizona Reps. Flake and Raul Grijalva, a Democrat, Politico reported.
When Obama made his way to the podium, he stopped by Giffords, gave her a long hug and kissed her cheek. Kelly also attended the address as a guest of first lady Michelle Obama.