The man tried to gain access to the kitchen
Police were forced to use CS gas to restrain the man when they entered the Zizzi restaurant in The Strand on Sunday evening.
A Metropolitan Police spokeswoman said the man was aged between 30 and 40 and that his injuries were self-inflicted.
The man was then taken to hospital in south London where his condition is stable. It is understood surgeons were unable to reattach his penis.
A spokeswoman for Zizzi said the man was not thought to have any connection with the restaurant.
She said: "At around 9pm on Sunday, a man walked into the Zizzi restaurant on The Strand, down the stairs to the basement restaurant area and tried to enter a kitchen.
"Members of staff stopped him, at which he ran into a second kitchen area.
"The man then picked up a kitchen knife and slashed himself across the wrist and groin areas before running back into the restaurant, where he continued to stab himself.
"This happened in a matter of seconds and was obviously extremely frightening and distressing for the many customers and staff in the restaurant at the time."She added: "Apart from the man, we understand that no-one else suffered any physical injuries."