"Governor Branstad and I still have a lot we want to accomplish on behalf you, the people of Iowa," she wrote in the email.
"That's why, after serious and thoughtful discussions with family, friends, supporters and constituents, I have decided to remain as Lieutenant Governor and will not seek a seat in the United States Senate."
Sources told NBC, which first reported Reynolds' decision, that she looks more likely to run to replace Republican Gov. Terry Branstad.
Reynolds had initially indicated she was interested in the seat but would not challenge Rep. Steve King (R) in a primary.
It's unclear whether King will launch a bid for Senate or defend his House seat, however, and his paltry fundraising sum for the first quarter — he brought in less than $100,000 by the end of March — has given some reason to believe he's leaning against what's likely to be a contentious fight.
Rep. Tom Latham (R) has also decided against a run.
Democrats already have a top recruit in Rep. Bruce Braley (D), who received Harkin's endorsement this week and leads nearly every potential Republican contender in every poll of the race so far.
Attention will now turn to a handful of other potential Republican candidates, including state Secretary of State Matt Schultz, former U.S. Attorney Matt Whitaker and state Secretary of Agriculture Bill Northey.