The designation is used for players who either test positive for the coronavirus or have had close contact with a person diagnosed with the virus.
Thielen had eight catches for 123 yards and two touchdowns in Sunday's 31-28 loss to the Dallas Cowboys.
Minnesota plays the Carolina Panthers in Week 12.
Unlike the traditional injured reserve designation, going on the reserve/COVID-19 list doesn't require a player to miss a minimum amount of time. The Detroit Lions, for example, put Matthew Stafford on the list earlier this month and activated him days later.
Were he to be absent against the Panthers, Thielen would be a massive loss for the Vikings.
The two-time Pro Bowler has caught 49 passes for 646 yards and an NFL-best 11 touchdowns. He has been particularly good in the red zone.
Thanks to first-round draft pick Justin Jefferson, Minnesota isn't missing Stefon Diggs as much as many thought the team would coming into the season. Jefferson's 848 receiving yards are first among rookie receivers.
Still, Jefferson and Thielen together have accounted for a little more than 60 percent of Kirk Cousins' 2,461 passing yards. Take one of those two out of the mix and it creates obvious problems for the passing game.
At 4-6, the Vikings have almost no margin for error in their bid to make the playoffs. Now, they might have to play without one of their top offensive players in six days' time.