Who knew first that Tom Riddle was the Heir of Slytherin: Tom Riddle or Dumbledore?

Question

While watching Harry Potter and the Half-Blood Prince this morning, my son asked a lot of plot questions, and one of his inquiries about Tom Riddle stumped me. So, here I am, asking all you fine people for help!

Who knew first that Tom Riddle was the Heir of Slytherin: Tom Riddle or Dumbledore?

I would love to see a definitive canon answer to this question, if at all possible.

There were obviously different clues dropped over the years that pointed toward Tom Riddle being the Heir of Slytherin. A few examples:

• Tom Riddle disclosed he could speak with snakes to Dumbledore when Tom was eleven and did not yet know the implications of being a Parselmouth.
• Merope Gaunt's Slytherin lineage.
• The diary Horcrux and the Chamber of Secrets.
• The basilisk and Myrtle's death.

Is it established in canon -- in reference to the Heir of Slytherin, Dumbledore, and Tom Riddle -- who knew what, and when? Who knew first that Tom Riddle was the Heir of Slytherin?

As I already mentioned, I'm looking for an answer grounded in canon -- the books, interviews and/or quotes by J.K. Rowling, or Pottermore. Speculative answers within the spirit of canon are totally fine by me. I did search the site to ensure this question has not been asked already and I didn't find anything -- if this is a duplicate, vote accordingly :)

Answer 1

First off, I'd like to dispute the nature of the question. Although Voldemort is the youngest known descendant of Salazar Slytherin it's pretty clear that other wizards have come from the same lineage. As a matter of fact, Voldemort wasn't even the rightful heir. That dubious honour fell to Morfin Gaunt, Marvolo's elder son.

The whole "Lord Voldemort : Heir of Slytherin" seems to have been something that Voldemort concocted on his own, as part of his attempt to rationalise his mixed parentage and to construct a mythos around himself. As such, Dumbledore couldn't have been the first to know that Riddle was styling himself thusly.

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That being said, it's reasonably certain that Voldemort was the first to learn about his true lineage as part of his research into his mother and father.

Dumbledore may have had his suspicions (due to the factors you've mentioned above) but it was definitely Tom Riddle who discovered his mother's name and learned of his family's connection to Salazar Slytherin;

Finally he was forced to accept that his father had never set foot in Hogwarts. I believe that it was then that he dropped the name for ever, assumed the identity of Lord Voldemort, and began his investigations into his previously despised mother’s family – the woman whom, you will remember, he had thought could not be a witch if she had succumbed to the shameful human weakness of death.

‘All he had to go upon was the single name “Marvolo”, which he knew from those who ran the orphanage had been his mother’s father’s name. Finally, after painstaking research through old books of wizarding families, he discovered the existence of Slytherin’s surviving line. In the summer of his sixteenth year, he left the orphanage to which he returned annually and set off to find his Gaunt relatives. - _{^{HP and the Half Blood Prince}}