Pastor, 70, who represents downtown Phoenix, was re-elected from Arizona's 7th District, which was carried by President Obama by a 45-percentage point margin, in 2012, making it likely a Democrat will succeed him in Congress, Roll Call said.
Roll Call said Assistant Arizona state House Minority Leader Ruben Gallego is likely to seek Pastor's seat in Washington.
Pastor was the first Hispanic elected to Congress from Arizona and served as Maricopa County Supervisor from 1976 to 1991.
"I have been in public office for 39 years and it's been a pleasure to serve the people of Arizona," Pastor said in an email statement. "After 23 years in Congress, I feel it's time for me to seek out a new endeavor. It's been a great honor, a great experience and a great joy for me to serve in Congress. I think it's time for me to do something else."