The Colorado Rapids goalkeeper first made the announcement via his Twitter account.
"It's been one heck of a ride," Howard told reporters on Tuesday. "This is something that's been on my radar for a number of years now, probably since I signed with Colorado. I knew that the length of the contract would take me to being 40, and it seemed like the right time. I've always had my sights set on this, and there are other things I want to do.
"For a lot of those reasons I wanted to make sure that this would be the end. The timing just makes a lot of sense to do it now before the season to get that out of the way so that it's not a distraction.
"That's what's most important to me. I feel great. It's not something I thought about last night. It's a decision I've been very comfortable with for quite a long time in my own head and heart. I feel good."