Beckham posted a photo of the fine letter Monday on Instagram. He can appeal the fine.
"14K for some [expletive emoji] pants that are not gonna protect me from anything," Beckham wrote on Instagram. "This [expletive] is ridiculous."
The NFL rule book states that players must cover their knee area with their pants when playing. His pants failed to cover his entire knee when he played in the Browns' loss to the Seattle Seahawks Oct. 13 in Cleveland.
The former New York Giants wide receiver was also fined $14,000 for unsportsmanlike conduct in Week 4. He was briefly removed from the Browns' Week 2 game against the New York Jets because he was wearing an unregulated visor on his helmet.
"I think it's crazy ... The NFL finds ways to just take it," Beckham said in early October. "They're going to take it from you. I understand a penalty, getting fined for that. But pants above the knees, a shirt hanging out -- and we're talking about a T-shirt that cost $5 to make, and you're getting $5,000 taken from you.
"Pads above your knees, and I'm like, 'You really think that this little knee pad covering my kneecap is going to affect, if somebody hits me, weighs 250 pounds, running 18 mph.' I don't do physics, but if I get hit, this is not going to protect me."
Beckham has 29 catches for 436 yards and a touchdown in six games this season. He had six catches for 101 yards in the Week 6 setback. The Browns (2-4) face the New England Patriots in Week 8 after a Week 7 bye.