Medics performed CPR on Hamlin on the field, surrounded by the other players, and he was then taken to hospital.
According to a statement from the NFL, his condition is critical.
The game was suspended and the NFL announced after just over an hour that it would not be resumed.
“Our thoughts are with Damar and the Buffalo Bills.
We will provide more information as soon as it is available,” the league said in a statement.
Damar Hamlin, who is 24 years old, collapsed shortly after a tackle situation with Tee Higgins of the opposing team Cincinatti.
Hamlin got up after the tackle and appeared to be adjusting his face shield when he suddenly fell backwards and lay motionless on the ground.
Several of the players were teary-eyed on the field.
- No one has experienced anything like this.
I haven't seen anyone like it either, said expert Troy Aikman on ESPN's broadcast.
It is not clear if the match will be resumed later.