A Miami federal judge set the plea change Monday afternoon for 47-year-old Rene Roberge of Sherbrooke, Que., who had previously pleaded not guilty.
Roberge's lawyer had previously said a plea deal was in the works to resolve the case without a trial.
Authorities allege Roberge flew to Florida after making contact on the Internet with undercover investigators posing as a 14-year-old boy and his father to arrange a sexual encounter.
Agents also allegedly found child pornography on Roberge's tablet computer.
Roberge is also accused of planning to travel to San Antonio, Texas, to have sex with a teenage boy.