Sub-Lt. Jeffrey Paul Delisle, 41, entered the guilty plea and skipped trial and will be sentenced at a date to be determined, the Canadian Broadcasting Corp. reported.
Delisle's lawyer, Mike Taylor, told CTV News his client made the decision to plead guilty two weeks ago.
Delisle is charged with a breach of trust charge under the Criminal Code and two counts of passing classified information to a foreign entity under the Security of Information Act.
The accusations date from July 2007 to January 2011 while Delisle was working at an intelligence facility in Halifax, the CBC said. He was denied bail and has been in custody since his arrest last January.
Prosecutors allege Delisle was paid $3,000 a month to share classified information with Russian intelligence agents, CTV said. That information was under a publication ban to avoid bias in his trial, which he waived.
The nature of information he sold wasn't divulged.
He is the first person to be charged under the security act, which was enacted after the Sept. 11, 2001, terror attacks on the United States.