Ben Patey, 33, was travelling home from work when he captured the amazing snap at the north London Underground stop.
He said: “I had just had a long day and I was waiting to jump on the train when I looked across and saw the nuns and the sign.
“I had to do a double-take. It was one of those strange but amusing moments.”
Seven Sisters is believed to take its name from a group of seven elm trees which were planted around a willow tree in the 14th Century.
It is thought they may have been planted by seven non-ecclesiastical sisters who were going their separate ways.