The couple sat for a while on the Diana seat, where Prince William's mother, the late Princess of Wales, had sat alone during a February 1992 visit.
The couple were accompanied by Archaeological Survey of India (ASI) officials during the visit. The British royals also saw the Yamuna river which flows at the rear of the monument.
They are to return to Delhi later in the evening on their way back to the UK.
The royal visit led to many tourists being held up at the western gate entrance. Many were unaware about the visit and were curious to know why they were not being allowed entry.