The Â£150m contract will involve constructing two new buildings and refurbishing existing buildings.
The first phase of the development, which is being built by McLaughlin and Harvey, is expected to be completed within the next two months.
The whole project is to finish by 2018.
It will serve around 15,000 students and staff.
Ulster University acting vice-chancellor, Prof Alastair Adair, said the awarding of the contract brings the campus "a step closer".
Colin Loughran, chief executive of Lagan Construction Group, said the project is "a landmark build in a key part of the city".
The campus development has sparked a series of planning applications for a student housing scheme in the area.
The second phase of the Ulster University's new Belfast campus will be built by a joint venture between Lagan Construction Group and the Portuguese firm Somague.