Marshall joins flyweight Nicola Adams and lightweight Natasha Jonas, who secured their qualification on Thursday, meaning there will be a full complement of British boxers when women make their historic Olympic debut in the sport.
Marshall will now face Azerbaijan's Elena Vystropova in the final as she bids to upgrade her silver medal from 2010.
She beat Russia's Nadezda Torlopova in her semi-final, taking the lead in the first round and never relinquishing it, winning 18-10.
Adams also progressed to the final beating Russia's Elena Savelyeva 11-6 and will now take on China's Ren Cancan as she seeks to add the world title to her European crown.
But Jonas settled for bronze after doing down to Russia's Sofya Ochigava 18-10 in her semi-final.
Ochigava will now face Irish rival Katie Taylor, as the three-time world and five-time European champion looks to arrive at the Olympics, where she is considered the best Republic of Ireland medal hope, with yet another title.