Knudsen, 23, has played in the Norwegian second and third divisions with Skiold and Notodden.
"Morten is a good prospect who I hope can break into the team this season," said Boothroyd.
Boothroyd added: "I have previously said it is important not to have too many players at the club but sometimes you have to grab opportunities when they come along.
"I believe we can mould him into another really good player for club."
Knudsen's arrival comes on the same day as 17-year-old defender Tom Bender signed a two-and-a-half-year professional contract with Colchester.
Source: BBC Sport