Johnson is the No. 7-rated draft-eligible offensive tackle on ESPN Mel Kiper Jr.'s latest top prospects list.
Johnson started 31 straight games at left tackle for the Seminoles, who were 10-3 this season and ranked eighth in the final Top 25 poll. He has won the Jacobs Blocking Trophy the past two seasons, which is given to the top offensive lineman in the Atlantic Coast Conference.
He said in a statement on Tuesday that he made the decision after further discussions with family.
Johnson is the second Florida State player to announce he is leaving early. Running back Dalvin Cook, who rushed for a school-record 4,464 yards during his career, announced his intentions on Dec. 31.