Government Chief Whip Gavin Williamson told the Daily Telegraph he keeps the spider - named Cronus - on his desk.
But how long Cronus will be living in the famous building is not clear.
A Commons spokeswoman said only guide dogs and sniffer dogs were allowed in Parliament - anyone bringing in another animal is "gently reminded of this policy and asked not to do it again".
The office of the Serjeant at Arms - which is responsible for maintaining order on the Parliamentary estate - would get in touch with anyone breaching the rules, the spokeswoman said.
But she would not comment on a report in The Sun that Mr Williamson had already been told by the Serjeant at Arms to remove the one-year-old spider, saying individual cases would not be discussed.
Mr Williamson, who is in charge of maintaining party discipline, told the Telegraph: "I've had Cronus since he was a spiderling, so I have a very paternal sort of approach. It's very much the same sort of love and care that I give to my spider as I give to all MPs."
He added: "Cronus is a perfect example of an incredibly clean, ruthless killer... absolutely fascinating to rear."
Other high-profile Westminster pets include Downing Street cat Larry, known unofficially as Chief Mouser to the Cabinet Office, and his Foreign Office counterpart Palmerston, while MPs and peers parade their pooches at an annual dog of the year show.
The Houses of Parliament are home to 650 MPs, about 760 peers - and, it has emerged, one tarantula.