5th June 2019, 11:24 | Leeds
The 24-year-old former Bradford Bulls forward is out of contact with the Giants at the end of the year and had several offers before opting to join the Rhinos on a three-year deal.
Leeds director of rugby Kevin Sinfield said: “We are delighted to have secured Alex on a long-term deal with the club.
“He is an exciting young English forward who has developed his game since returning to Super League with the Giants.
“Despite only being 24, he has over 100 senior appearances already and I am sure he will continue to progress within our system.”
Mellor has made 63 appearances for the Giants since moving to the John Smith’s Stadium in 2017 and has scored seven tries in 14 appearances so far this year.
Mellor said: “I am really grateful for the opportunity to join Leeds Rhinos and I can’t wait to get started next season.
“I have had two fantastic years at Huddersfield and I am grateful for everything they have given me.
“Leeds is a great club and, while they are having a tough season so far, hopefully I can play my part in helping the team get back to where they belong.
“It is a huge club and I am excited about being part of the process to rebuild the club’s fortunes on the field.
“I live locally and, while I had a couple of offers to move elsewhere, Leeds was always my first choice.”