Grimm, who served two terms in Congress before resigning in 2015, easily hurdled the 1,250-signature mark to file for the ballot Tuesday, the first day of eligibility to file in New York, gathering more than 3,000 signatures from people in the district.
The highly-anticipated primary for the Staten Island seat is slated for June 26.
The race is already simmering on the Republican side.
Last week, Donovan publicly speculated whether Grimm was behind an ethics complaint targeting the incumbent by alleging that he got his partner’s son out of a drug bust.
The Office of Congressional Ethics received a complaint that Donovan used his position as a member of Congress to get Timothy O’Connell, his girlfriend’s son, out of a heroin-related arrest.
Donovan denied the claims last Monday at a news conference, NBC News New York reported.
“I don’t know if Michael Grimm is the anonymous source, but someone had to instruct this individual how to file a House Ethics Committee complaint,” Donovan said.
“Someone who may know [how to file an ethics complaint] is a person who has had allegations made against them for threatening to throw a reporter off the rotunda after the State of the Union address, for FEC filings,” he said. “So someone who has had complaints against them filed might know how to do it.”
Donovan was referring to an incident in which Grimm threatened a New York reporter in the Capitol.
Grimm resigned from office in 2015 after being indicted for tax evasion, hiring undocumented workers, and perjury under oath.
Those charges led to a eight-month stay in federal prison for tax fraud after Grimm admitted he had hidden more than $900,000 in receipts at his Manhattan restaurant, Healthalicious. He was released in May 2016.
Grimm has established himself as the insurgent in the race, ready to reclaim his former seat with an aggressive campaign tone.
Former White House chief political strategist Steve Bannon endorsed him in October.
Whoever emerges from the June primary should be favored in the general election in November. Inside Elections with Nathan L. Gonzales rates the race Likely Republican. But the Democratic Congressional Campaign Committee named Army veteran Max Rose to its Red to Blue program for top recruits earlier this year.
President Donald Trump carried the district by 10 points in 2016 while Donovan was winning a first full term by nearly 15 points.
Watch: What to Expect, and Not Expect, When the House Returns