She took to the water under the light of the full moon at 5.15am on Wednesday from Fort Le Marchant on the north coast.
She swam more than 30km in just five hours and 25 minutes – the fastest time anyone has ever swam the distance without a wetsuit.
Miss Riley landed at Trois Vaux Bay in Alderney with a crowd of gannets swooping over the bay to greet her.
She collected a souvenir pebble on shore, posed for a brief photo, then had a well-earned lunch at Cantina No. 6, overlooking Braye Beach.
‘It was a very special day yesterday, not least because Delphine had set her sights on this after finishing her swim from Guernsey to Sark as the youngest swimmer last year, but also because Delphine’s great-grandparents and grandfather came from Alderney and we know they would be immensely proud of her,’ said her mother, Justine.
Miss Riley said she had to start the swim in the dark because of the tide times, and it was spooky to be in the water with only the moon above.
Her friend Terry Wallhead swam alongside her until the sun rose over the sea at 7am.
‘As her mother, I’m immensely proud of Delphine,’ said Mrs Riley.
‘I got quite emotional when she landed in Alderney.
'The support crew dropped me at the end so we could be together when she got to shore, and it was so special.’
Miss Riley spent the summer training for her big swim. She completed the triple Herm challenge with Guernsey Swim Adventures, which meant she swam to Herm, around Herm and back to Guernsey.
She also did a 10k south coast swim, a swim to Sark and many more hours training with friends to prepare herself for her biggest challenge yet – swimming from Guernsey to Alderney.
She said the most difficult part of the journey was early on, as she swam through choppy waters going past the reefs near Guernsey. However, she said she remained determined throughout the whole journey, and never considered giving up.
Miss Riley’s support team included friends Mr Wallhead and Jan Turton, who both swam alongside her at intervals; her father George who was beside her at all times in a kayak; and a guide boat piloted by her brother Tim and John Chandler.
Mrs Riley and friend Geoff Le Page were on board the guide boat as well to provide encouragement.
Miss Riley said she enjoys breaking records, but the records are merely an added bonus. She simply has a deep love of swimming in the sea. Now that her big accomplishment is behind her, she said she feels a bit sad that it is all over.
Miss Riley swims competitively with the Beau Sejour Barracudas, so she is keen to get back in the water and work on new and different challenges over the winter.
Last year she broke the record for being the youngest person to swim from Guernsey to Sark.
This year, she broke two records for her swim to Alderney.
But next year, she said, her big challenge would be to conquer her GCSE exams.