The 23-year-old has moved to the Scottish Premiership after rejecting the chance to sign a new deal with Irish League side Cliftonville this summer.
Boyce has scored 60 goals in the past two title-winning seasons at Solitude and will now return to full-time football for the first time since a dream move to Werder Bremen turned sour in 2011.
He had spoken to several clubs in England and Scotland but was persuaded to join the Staggies.
County manager Derek Adams said: "We have been tracking Liam's progress for a while now and are delighted to have completed his signing.
"Liam had opportunities in Scotland and England but after visiting us last week has chosen to join Ross County.
"He is a proven goalscorer and has recently been called up again to the full International team."
Boyce has made four appearances for Northern Ireland and was included in the squad for their recent friendlies in Uruguay and Chile but did not feature in the matches.