The Swiss-based private equity firm bought the site near Aldgate station out of receivership last month for around £55m.
It plans to start demolishing the existing building, which is vacant, at the end of June and start construction in early 2016.
The new 14-storey office block, designed by architects TP Bennett, will cost around £200m to develop and is expected to be completed in the first quarter of 2018.
Stuart Keith of Partners Group said: “The recent rebound in the UK economy has created a significant supply-demand imbalance in the City in terms of Grade A office space, with vacancy levels close to historic lows and rents steadily climbing. 80 Fenchurch will be perfectly placed in terms of location, timing and quality of build to capitalize on this opportunity".