Astonishingly the practice, which was the only British-based architect to make it into the final six, beat Rem Koolhaas, Jean Nouvel, Daniel Libeskind, Herzog & De Meuron and Massimiliano Fuksas to scoop first prize in the contest.
The remarkable victory means RMJM will now get its hands on designing a 300,000m 2commercial complex, including a 300m-tall office tower, next to the city's famous Bolsheokhtinsky Bridge.
However, RMJM will first have to deal with the growing number of people opposed to the principle of a tower on the riverside plot.
Over the last few weeks all the entrants have come under increasing criticism from a gaggle of top-level opponents to the massive £324 million office scheme - including the head of St Petersburg's State Hermitage Museum.
Earlier this month, Hermitage director Mikhail Piotrovsky blasted every one of the competition entries - all of which featured enormous skyscrapers - because of the potential impact on the historic city, which was once Russia's imperial capital.
The former head of the city's architectural protection committee, Nikita Yaveyn, also came out against the schemes, and has been quoted as saying that if any of them were ever realised the 'only solution [would] be to call in terrorists to blow it up'.