Charlotte, eleventh in line to the Monegasque throne, is the second child of Princess Caroline of Hanover and the late Italian businessman Stefano Casiraghi, who died in 1990.
The mother-of-two reportedly met Dimitri in Paris shortly after the end of her relationship with Italian filmmaker Lamberto Sanfelice.
It is thought Charlotte and Dimitri will take the opportunity to baptise their seven-month-old son Balthazar on the same day.
The couple will likely follow the lead of other Monegasque royals - including the bride's brothers, Andrea and Pierre - and hold a second, more lavish celebration to mark their nuptials at a later date.
Charlotte, 32, and Dimitri Rassam, 37, will marry in the state rooms of Monaco's royal palace on Saturday, reported today.
The sleek black silhouette is adorned with sequinned hearts and stars to create a glamorous yet playful feel.