Middleton, 32, earlier fueled rumors on Monday when she appeared at a charity fundraiser and notably kept her left hand hidden from photographers.
Attending a gala for the British Heart Foundation, the royal sister-in-law stunned in a floral L.K. Bennett gown alongside Princess Eugenie and Sarah, Duchess of York – but made sure the ring finger of her left hand was never seen.
Middleton and Matthews first dated in 2012 before subsequently splitting – with the party-planning author later dating Nico Jackson for three years.
Three months after Middleton and Jackson parted ways, Matthews, 40, was spotted by Pippa's side during a St. Barts vacation with her brother, James Middleton.
Middleton and Eton-educated Matthews, who runs Eden Rock Capital Management, have also vacationed together with her family. Last August, Middleton and her family spent time at his parents' hotel, Eden Rock, in St. Barts.
Matthews also joined the younger Middletons when they took part in a grueling 47-mile swim-run in Sweden last September.
Most recently, the couple were spotted enjoying Wimbledon together.