The 23-year-old is out of contract at Boundary Park and had also attracted interest from Dundee United.
Agent Lee Payne told BBC Sport: "He has agreed terms in principle with Motherwell and will head back there after the weekend to complete the deal.
"Mark McGhee has made a big impression on him. Now Chris has the opportunity to make his mark in the SPL."
Payne said that Dundee United manager Craig Levein had also impressed Porter but that Fir Park was just a little closer to the Englishman's home.
The Fir Park club missed out on Craig Dargo, with the former Inverness striker opting to join St Mirren instead.