In April 1792, he married the 26-year-old Simone Évrard in a common-law ceremony on his return from exile in London, having previously expressed his love for her. She was the sister-in-law of his typographer, Jean-Antoine Corne, and had lent him money and sheltered him on several occasions.
Marat was in his bathtub on 13 July, when a young woman from Caen, Charlotte Corday, appeared at his flat, claiming to have vital information on the activities of the escaped Girondins who had fled to Normandy. Despite his wife Simonne's protests, Marat asked for her to enter and gave her an audience by his bath, over which a board had been laid to serve as a writing desk. Their interview lasted around fifteen minutes. He asked her what was happening in Caen and she explained, reciting a list of the offending deputies. After he had finished writing out the list, Corday claimed that he told her, "Their heads will fall within a fortnight". A statement which she later changed at her trial to, "Soon I shall have them all guillotined in Paris". This was unlikely since Marat did not have the power to have anyone guillotined. At that moment, Corday rose from her chair, drawing out from her corset the five-inch kitchen knife, which she had bought earlier that day, and brought it down hard into Marat’s chest, where it pierced just under his right clavicle, opening the carotid artery, close to the heart. The massive bleeding was fatal within seconds. Slumping backwards, Marat cried out his last words to Simonne, "Aidez-moi, ma chère amie! ("Help me, my dear friend!") and died.
Corday was a Girondin sympathiser who came from an impoverished royalist family—her brothers were émigrés who had left to join the exiled royal princes. From her own account, and those of witnesses, it is clear that she had been inspired by Girondin speeches to a hatred of the Montagnards and their excesses, symbolised most powerfully in the character of Marat. The "Book of Days" claims the motive was to "avenge the death of her friend Barboroux". Marat's assassination contributed to the mounting suspicion which fed the Terror during which thousands of the Jacobins' adversaries – both royalists and Girondins – were executed on supposed charges of treason. Charlotte Corday was guillotined on 17 July 1793 for the murder. During her four-day trial, she had testified that she had carried out the assassination alone, saying "I killed one man to save 100,000."
After Marat's death, his wife may have sold his bathtub to her journalist neighbour, as it was included in an inventory of his possessions after his death.