McDonald, 35, had 489 points in 685 games with the Anaheim Ducks and Blues, but concussion issues have plagued him over the years.
"I'm fortunate to get out now," McDonald told Andy Strickland of the website TrueHockey.com. "I know I could play two or three more years, and I love the game of hockey, but health-wise I know I shouldn't be playing."
McDonald spent the past six seasons with the Blues, and had 21 points in 37 regular-season games this season but was held without a point in the Blues' first-round playoff loss to the Los Angeles Kings. Once that series ended, McDonald said he knew it was time to put away the skates.
"The last few years, too much of the focus became worrying about the next hit," McDonald said. "I was always thinking about it."
An undrafted free agent, McDonald signed with the Ducks in April 2000 after a four-season career at Colgate University. He scored at least 20 goals twice in seven seasons with the Ducks, and had 14 points in 21 Stanley Cup Playoff games in 2007 to help Anaheim win the championship.