Mrs May will announce the moves in Nigeria, on the second day of a three-day trip to Africa.
Modern slavery is a scourge she has focused on since she was Home Secretary.
Nigeria is the fifth-largest country of origin for victims of modern slavery in Britain, with men, women and children tricked into coming to Europe with the risk of abuse, trauma, prostitution, forced labour and death.
Ahead of her visit, Mrs May said: “Modern Slavery is one of the greatest human rights abuses of our time and the UK is a world leader in making it an international mission to end this heinous crime.
“Today we are stepping up our partnership with Nigerian authorities to find traffickers and bring them to justice.
“And because this is an international problem which needs international response, we are also launching a new project with France to strengthen border cooperation to prevent trafficking along key migration routes towards Libya and Europe.
“But as well as targeting the smugglers and traffickers that cruelly exploit people for financial gain, it’s vital that we support the victims who have suffered enormous trauma and are at high risk of being re-trafficked, and that is an important part of the support we are announcing today.”
The plans include support to help up to 1,700 migrants and modern slavery victims returning to Nigeria from Libya with counselling to deal with the distress of their ordeal, and training in business and vocational skills to help them get jobs and reintegrate into their communities.
These traumatised people are often at high risk of being retrafficked.
The UK and France will help the governments of Nigeria and Niger strengthen their border cooperation, to prevent trafficking on one of the main migration routes towards Libya and Europe.
A UK-funded border force at Nigeria’s Lagos airport this year helped secure the first prosecution of a British national for trafficking under the UK Modern Slavery Act, with Josephine Iyamu jailed for 14 years in Birmingham in July.