Taylor flew out to the USA last week for talks with the Major League Soccer club.
The 30-year-old is understood to have passed a medical and agreed terms over a long-term move to at Providence Park.
Newcastle played Portland, the reigning MLS Cup champions, in pre-season a year ago.
United, then coached by Steve McClaren, were beaten 4-3 by the club.
The former England Under-21 international made 268 appearances, and scored 15 goals, for the club he joined as a nine-year-old.