The deal is with ITL Trading, which will distribute it into Nigeria.
It covers up to 350,000 tonnes of POLY4 per year for the seven years following the start of production.
ITL Trading, which is one of the largest suppliers of fertiliser to Nigeria, is a global trading company with an active presence in Europe, Africa, the Middle East and Asia.
It specialises in supplying Nigeria and West African clients with globally sourced products and services that are critical to laying the foundations for a modern economy.
Chris Fraser, managing director and CEO of Sirius, said: “Africa is a huge potential market for POLY4 and we are very pleased to establish our initial footprint in Nigeria, which is the largest market in West Africa.
"Nigeria is positioned as the key market for fertiliser growth in the region and we believe ITL Trading will be a fantastic long-term partner for the company."
Mr Fraser said Sirius is making good progress on all aspects of the mining project.
Analyst Paul Smith at WH Ireland said: "Today’s announcement is a small, but welcome addition to the offtakes already agreed by Sirius.
"This is a new market for POLY4, but again demonstrates the efficacy of the natural product in different climates, countries and for different crops."
Sirius is in active discussions with other fertiliser players in Europe, India and Brazil to support Stage 2 financing, which is expected to be completed by the end of the year.
Mr Smith said: "We anticipate further offtakes in due course. This is an interesting update from Sirius today in our opinion and along with the milestones being made on site in the UK shows the progress being made by the company as it bids to bring all strands of this important build together."
Preetpal Walia, managing director of ITL Trading, said: “The Nigerian fertiliser market is undergoing strong growth and there is tremendous potential for multi-nutrient products which can deliver balanced fertilisation.
"We see POLY4 as being an ideal product to meet this opportunity and are very pleased to be entering into this long-term arrangement.”