The 22-year-old, who has appeared twice for Spurs' first team this term, is now committed to the club until the summer of 2014.
But Obika's immediate future lies in the Championship with Charlton, who have brought him in as a replacement for the departed Bradley Wright-Phillips.
Manager Chris Powell said: "After losing Bradley Wright-Phillips I just felt we were short in the striker department.
"Jonathan is a player we have kept an eye on this season in a number of games. I saw him when he played against us and he has an eye for a goal.
"He is a player full of potential and it is a chance for us to have a good look at him. Hopefully he will come into the group and make an impact."
Obika has previously spent time in the Championship with Crystal Palace, though his most recent loan spell was with League One Yeovil.