"A lot has happened over the past year. We cannot change that," Giffords said in a video on her website. "I don't remember much from that horrible day, but I will never forget the trust you placed in me to be your voice."
"Thank you for your prayers and for giving me time to recover. I have more work to do on my recovery. So to do what is best for Arizona, I will step down this week."
Giffords is expected to attend the State of the Union Address Tuesday and then submit a letter of resignation to House Speaker John Boehner and Arizona Gov. Jan Brewer.
The state will hold a special election to fill the Democrat's seat later this year.
