"I never really counted the Blues out until I put pen on paper," he said Tuesday.
The defenseman had spent his entire 12-season NHL career with the Blues, the last four as captain. He became the first player in their history to raise the Stanley Cup after they won it in 2019.
His wife, Jayne, is from St. Louis. They have four children.
"It was difficult," Pietrangelo said. "It wasn't as easy as maybe some people think it was for us. When you have your roots tied here and you've been here for so long, it's definitely a difficult decision. But I tried to take as much emotion out of it as I could."
Pietrangelo said the goal always was to re-sign with the Blues, but the sides couldn't agree to terms before the free agent market opened at noon ET on Friday. He didn't close the door on them when he heard they had signed defenseman Torey Krug to a seven-year, $45.5 million contract Friday night.
"[Blues general manager Doug Armstrong] saw an opportunity to get an elite player, and he did that," Pietrangelo said. "I guess I was surprised. I wasn't really paying attention either."