It has contracted McLaughlin & Harvey Ltd for engineering, procurement and construction of the facility.
The scope of the project includes product storage facilities for approximately 250,000 tonnes of product, ship loading equipment, ship outload infrastructure, and the final product screening facility.
Sirius noted that the lump-sum payment for the work will be in line with most recent estimates, stated in September, and, the programme will fit into the existing project schedule.
"We are pleased to have entered into another significant construction contract for the ongoing construction of our world leading polyhalite project,” said Chris Fraser, Sirius managing director.
“In 18 months we have made great progress and are now nearing the completion of the procurement programme to support our stage 2 financing process.
"The construction of our port infrastructure is another example of the level of investment and skilled job creation we are delivering in both Teesside and the UK as we develop a project that has the potential to make the UK a world leader in the fertilizer industry."
Sirius also told investors that its stage 2 financing efforts are now nearing completion.
The financing is expected to cover the major construction packages with only the mineral transport system fit-out scope remaining.
STRABAG has been identified as a preferred contractor for the MTS fit-out, and, negotiations are said to be in the final stages.