Anthony has not featured in the league in over a year, since a short-lived spell with the Houston Rockets came to an end.
However, the 35-year-old has played over 1,000 games in a career that has spanned 16 seasons since he was selected with the third overall pick in the 2003 draft.
Anthony has previously played for the Denver Nuggets - the team who drafted him - as well as the New York Knicks and the Oklahoma City Thunder.
"Carmelo is an established star in this league that will provide a respected presence in our locker room and a skill set at a position of need on the floor," said Neil Olshey, Portland's president of basketball operations.
Anthony will wear number 00 for the Trail Blazers, who reached last season's Western Conference Finals but are struggling at 5-9 this year.
The NBA's scoring champion for the 2012-13 campaign has been activated in time to feature against the New Orleans Pelicans.