"Kit has thoroughly earned the opportunity to take Fulham from here as our first-team manager," Fulham owner Shahid Khan told the club website.
"We've been a different football club, on and off the pitch, since Kit stepped in as our caretaker manager in September. His passion for Fulham is evident and contagious, and I believe the promise ahead of him is extremely bright."
Symons's appointment comes a little under 24 hours after his side threw away a 2-0 half-time lead and went down 5-2 to Derby County in the fourth round of the Capital One Cup.
His first game in charge as full-time manager will be a trip to Wigan Athletic on Saturday.