Märta Allertz (1628 – before 1677), often wrongly referred to as Brita Allerts, was the royal mistress of Charles X Gustav of Sweden.
Allertz was the daughter of the wealthy Stockholm city councillor Claes Allerts (d. 1650) and the successful wheat merchant Britta Jacobsdotter.
He spent his first three years with his mother, was in 1650 entrusted to Magnus Gabriel De la Gardie and ennobled in 1674.
In contrast to other mistresses of Swedish royalty, Allertz did not marry directly after the relationship was broken.
In 1665 Allertz personally was granted eight estates by the Queen Dowager Hedvig Eleonora and she was by then no longer under her mother's guardianship, but married and with children.