The winning design – Sacred Grove – features a roof garden with a grove of 37 trees on top of a shady courtyard area with a central water feature.
The garden, which also won a gold medal, was praised by judges for its "exploration of light and shadow and the basic elements of waters, light and air, and intelligently addressing issues such as drainage, sustainability and the greening of the city.
Wilson said the duo were "over the moon"."Singapore Garden Festival is one of the most prestigious in the international horticultural calendar. Creating this design has given us the scope to experiment with our passion for contemporary garden design while addressing the requirements of a high-rise city – something we increasingly need to think about as our urban populations increases."
McWilliam said designing and creating Sacred Grove had been an extraordinary event.
He added: "The dynamic structure and planting design combine with a functional and cool space and create something very practical for the Singapore lifestyle."
The pair also worked together on the Cloudy Bay garden at RHS Chelsea this year, which won a silver gilt medal.
Singapore Garden Festival runs until Sunday at Gardens by the Bay, the award-winning 101 hectare public park and gardens were designed by Grant Associates and Gustafson Porter.