The royal couple were guests at the ceremony, which took place at Chelsea Old Church near the River Thames.
The occasion may well have been a trip down memory lane for the Duchess, as the church is situated on the very same road as the Middleton's London home, where Kate shared a flat with Pippa in the years she lived in the capital after graduating from St Andrews.
Kate, who is currently five months pregnant with her and William's second child, looked elegant in a floor-length black lace gown with full-length lace sleeves, most likely the same one by designer Diane Von Furstenberg that she opted to wear to the star-studded Royal Variety Performance last month. For the wedding, the Duchess accessorised with a small sparkling black clutch and black heels, wrapping up against the chill with a red tartan scarf that was draped over her shoulders. She wore her brunette locks loose over her shoulders.