Kun Shan Chun, a naturalised US citizen born in China and known as Joey, admitted in court to sending sensitive information to a Chinese official.
US Attorney Preet Bharara said he had threatened the country's security through a "duplicitous" betrayal.
Chun "deeply regrets" what he did and loves the US, his lawyer said.
While he could face a sentence of up 10 years in prison, the government and defence agreed a sentence of between 21 and 27 months "would be appropriate", the New York Times reports, citing the plea agreement.
The Chinese Embassy in Washington is yet to comment, the Associated Press reports.
Chun, 46, worked for the FBI as an electronics technician from 1997 until his arrest in March.
He was introduced to a Chinese government official during a trip to Italy and France in 2011, the Department of Justice says, and they continued to meet privately abroad.
He is accused of having covered up his connections with at least one Chinese government official and a Chinese technology company, Zhuhai Kolion, in which he had an "indirect financial interest", according to the justice department.
Court papers in New York said in 2013 he sent an unnamed Chinese official a chart showing the FBI's organisational structure and two years later sent photos of restricted documents about surveillance technologies.
Chun also provided the identity and travel schedule of an FBI agent, Assistant US Attorney Emil Bove said.
He was rewarded financially and through perks such as holidays and hotel stays, according to authorities.
According to the complaint, he was caught after offering to help an undercover agent transmit sensitive government information to his Chinese associates.

The group, believed to be the first all-amputee team to undertake the challenge, set off from the Canary Islands on 20 December.
Two of the rowers have reportedly struggled to eat anything since they set off, a spokeswoman said.
They have also been sent "flying off their seats", by heavy sea swells.
Lee Spencer, Nigel Rogoff, Paddy Gallagher and Cayle Royce, aged 29 to 56, were among 26 crews which set off from La Gomera to Antigua in the Caribbean.
Cayle Royce - 29, from Dartmouth. Suffered serious injuries serving in Afghanistan
Paddy Gallagher - 30, from Cambridgeshire. He was injured in Afghanistan while serving with the Irish Guards
Nigel Rogoff - 56, from Hereford, who lost his leg while taking part in an RAF parachuting display
Lee Spencer - 46, from Yelverton in Devon. He lost a leg when he was struck by debris when he stopped to rescue a seriously injured motorist on the M3
Mr Spencer, who spoke to his wife, Claire, on BBC Radio Devon, wished his family a Merry Christmas and said he was "really looking forward" to seeing them at the finish in Antigua.
Mrs Spencer told him he was "mad", but she was "very, very proud" and she would "raise a glass, or five, [to him] in the pub".
They aim to row the 3,000 miles in under 55 days, but with another 2,453 miles to go, the team has reported on Facebook that "sea sickness has well and truly kicked in with two members of the team struggling to eat anything since they set off".
They also report that the strong north-easterly winds have have also brought a "new challenge" with a battle between the swell and the oars.
A spokeswoman said: "The seas are quite rough and the oars keep pushing into their prosthetics.
"The big swell is also putting them off course so they are hoping for calmer conditions to get back on track."
The Talisker Challenge is branded as "the world's toughest row".
The spokeswoman said: "The first two weeks are known as hell on earth and it takes that long to adjust before they get in their stride.
"It's a long journey ahead. This is day four and there are at least another 45 days ahead."
The crew members come from Devon, Hereford and Cambridgeshire.
An FBI employee with top secret security clearance has pleaded guilty to acting as an "agent of the Chinese government", the US says.
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Four ex-serviceman attempting to row across the Atlantic have been hit by severe sickness since setting off.