The Dockers will lodge 'relevant documentation' with the AFL to confirm the signing of the 27-year-old unrestricted free agent, who has played 157 games for Melbourne.
Sylvia told Melbourne of his intention to leave the club at AAMI Park on Thursday morning.
Melbourne manager of football operations Josh Mahoney said the Demons had offered Sylvia a deal similar to Fremantle's.
"We offered Colin a contract, which we've been informed is of similar terms to the Fremantle contract," Mahoney said.
"But as an unrestricted free agent, he had a choice to make, and made his decision to accept Fremantle's offer. We thank Colin for his time at Melbourne and wish him well for the future."
Mahoney said he expected Sylvia to remain a Dee under new coach Paul Roos, but understood his decision to move on.
Taken with pick three in the 2003 national draft, Sylvia said moving to the losing grand finalists was the toughest decision of his career.
"This footy club has played a big part in my life," Sylvia said.
"This is the hardest decision I've made in my footy career to leave a club after 10 years.
"The Melbourne Football Club will always be in my heart. It has made me who I am today and I thank for them for that.
"I would also like to thank my team-mates over my journey at the club and the Melbourne supporters, they have been great for me and it's really sad to leave.
"I will always consider myself a Melbourne person. I leave on good terms and clearly this was a tough decision, given the positive moves the club has made in recent times.
"But Fremantle are a good fit for me, based on where I'm at with my football career."