Morris revealed talks with United boss Jackie McNamara sealed the deal.
He said: “After speaking to the manager I was set on joining United. This is a great club which showed last season that it is capable of producing one of the best teams in Scotland and I want to play my part in improving them further.
“Dunfermline were great for my career and I now want to continue my progress and coming to Tannadice is a great move for me.
“There are many talented footballers here and I am looking forward to playing alongside them.” McNamara said: “We have been after a commanding centre half and once Callum became available I acted quickly to bring him here.
“He is strong, aggressive and the right age for us. Despite interest from other Scottish Premiership clubs, Callum wanted to come to Dundee United and he will be an excellent addition to our team. I am delighted to have signed him.”
Morris started his career at Newcastle progressing through the youth academy and then joined Blyth Spartans, Hayes and Yeading United before securing a move to Dunfermline in 2012 where he made over 50 appearances. He rejected the contract offer available at the end of the season.