An Air Carrier pilot flew off course when they programmed the fix DENTT in the FMS instead of the correct fix DENNT.

Narrative

We were flying Aircraft X ORD to ZZZ. I programmed the FMS at the gate and checked the points against the flight plan because it was too early to get the clearance. First Officer (FO) came later down and he requested the ATC clearance via ACARS. We printed it out and check the clearance vs the flight plan I had put it the box and verified it against the Jeppesen charts. We departed on time pushed back and took off Runway 22L in ORD; Departure gave us a climb clearance and told us to go direct fix DENNT. ATC said a few minutes later Aircraft X verify you are going direct DENNT. I looked down at the FMS box and it had direct to DENTT. We then corrected the box with the correct spelling and flew to ZZZ without further problems.I had programmed the FMS with the fix DENTT instead of the correct DENNT. I check it against the clearance and the Jeppesen charts but never caught my misspelling by 1 letter till ATC said something. I should have slowed down and paid closer attention to the exact spelling of the fixes. I have flown over the DENNT fix hundreds of times without any problems.