The incident took place at Brunel University in Uxbridge between 19:30 BST on Saturday and 01:30 on Sunday. The man was held on Tuesday afternoon on suspicion of public order offence but has been bailed.
On Tuesday a student apologised for a “chainsaw-wielding clown” prank filmed on the university grounds.
The self-described YouTube prankster, who said he was a media studies student and gave his name as Kenny, told BBC Radio London he was “only chasing his friends” in a video shot on Brunel University campus.
A man in his 50s was also arrested by police in Hayes, west London, on Tuesday after reports of a man in clown mask possessing what looks like a firearm in a car.
Police said they were assessing three incidents for potential criminal offence.
The Met said it had received several reports of “killer clowns” frightening people across London since last week.
Commander Julian Bennett said: “We are currently assessing a small number of reported ‘killer clown’ incidents in London, three of which meet the threshold of a criminal offence.
“Our primary responsibility is indentifying and investigating those incidents where a criminal offence has occurred.
“However, anti-social behaviour can leave people feeling scared, anxious and intimidated and I would urge those who are causing fear and alarm to carefully consider the impact their actions have on others.”
Officers will carry out “additional reassurance patrols” in the run up to Halloween, he added.
6 October: A man walking towards Latimer Road Underground station in west London was confronted by a man wearing a clown mask and carrying a hockey stick. The suspect jumped out from behind shrubbery and chased the victim. No arrests have been made.
8-9 October: Reports are received of a man in a clown costume deliberately frightening several people at the Brunel university campus. A 19-year-old man was arrested on suspicion of a public order offence. He has been bailed until later this month.
10 October: Reports of a woman being chased by a man wearing a clown mask with a knife in Marlborough Road, Islington. Officers said the victim was walking when she saw the suspect on a bicycle and that he followed her before fleeing the scene in the direction of Hornsey Road. No arrests have been made.
Manor House Gardens, Hither Green: A 13-year-old girl said she was chased by someone wearing a clown mask and wig, and possibly carrying a small penknife. No arrests have been made.
11 October: Police were called to Bonham Road in Dagenham to reports of two children on their way to school seeing a person wearing a clown mask. The children ran away.
Church Green in Hayes: A man in a car wearing a clown mask and possessing what appeared to be a firearm reported to police at 15:45 BST. Heathrow Armed Response Unit responded and stopped a car in Uxbridge Road in Hillingdon. An imitation firearm was recovered and a man in his 50s was arrested on suspicion of possessing an imitation firearm.