The 12-year-old, second in line to the throne, was seen singing the hymn "Abide with Me" alongside his mother, the Princess of Wales.
The King and Queen were also in attendance at the event, which commemorates those who lost their lives in service.
This year's Festival of Remembrance marked the 80th anniversary of the end of the Second World War.
The event featured musical performances from Sir Rod Stewart and Sam Ryder, and included a rendition of "God Save the King" following a two-minute silence.