The 29-year-old Sinclair, from Burnaby, B.C., was assigned to Portland on Friday as part of the allocation of national team players — including 16 Canadians — for the new eight-team NWSL.
Thorns will be much like a hometeam for Sinclair, who starred for the University of Portland.
Canadian goalkeeper Karina LeBlanc and American Tobin Heath were also among the seven players assigned to Portland, the team that had social media buzzing after the allocation announcement.
"I don't think I would be crowning any champions this early, but I can understand why people would say Portland's got a couple of especially high-profile goalscorers," U.S. Soccer president Sunil Gulati said on a conference call. "But the northwest was popular with our players, in some cases because they've lived there, in other cases because they went to college there (as in Sinclair's case).
"Not an easy task but we tried to balance it as much as we could."
The Thorns were getting lots of attention on Twitter, one fan posting: "I'm actually freaking out right now that Alex Morgan and Christine Sinclair are going to be playing for the Portland thorns. #YES"
Another added: "But on a serious note the Portland Thorns are stacked..."
Sinclair was among 16 Canadian players, plus 23 Americans and 16 Mexicans assigned to rosters for the new women's league that will kick off this spring.
Players were allowed to specify their preferred destinations and the clubs also provided their preference for specific players or qualifies of players desired.
The Canadian Soccer Association is picking up the tab for its national team players. U.S. Soccer is funding 23 of its international players while the Federation of Mexican Football will pay the freight for 16 of its players.
Other Canadians who found out where they'll be playing were Adriana Leon and Rhian Wilkinson (Boston Breakers); Erin McLeod and Carmelina Moscato (Chicago Red Stars); Desiree Scott and Lauren Sesselmann (FC Kansas City); Kaylyn Kyle and Emily Zurrer (Seattle Reign FC); Sophie Schmidt and Melanie Booth (Sky Blue FC); Lori Lindsey and Robyn Gayle (Washington Spirit); and Bryana McCarthy and Jodi-Ann Robinson (Western New York Flash).
Sinclair led Canada to a bronze medal at last summer's London Olympics in spectacular fashion, scoring a tournament-record six goals to win the Golden Boot. The winner of the Bobbie Rosenfeld Award as the 2012 Canadian Press female athlete of the year set a national record with 23 goals in 22 matches in 2012.
At Portland, Sinclair led the all-time NCAA Div. 1 scoring record with 39 in her senior season, and capped her college career with two goals in a 4-0 win over UCLA in the NCAA championship game.
She also won the MAC Hermann Trophy as the NCAA's top women's soccer player back-to-back years, and went on to claim the Honda-Broderick Cup as the top female athlete in any NCAA sport.
The 23-year-old Morgan led the U.S. in scoring last year with 28, and helped the United States to gold in London.
The 16 Canadians — 13 of which were members of Canada's London Olympic team — were chosen by national team coach John Herdman.
All 18 U.S. players on the gold medal-winning 2012 Olympic team are included in the American allocation.