Dideriksen out-sprinted Sofia Bertizzolo (Italy) and Agnieszka Skalniak (Poland) to win her second rainbow jersey.
Alex Manly was the top finishing Australian, finishing 11th in the bunch sprint. Newly crowned time trial world champion Macey Stewart finished 35th, 20 seconds down, with Anna-Leeza Hull finishing 52nd.
The peloton remained together for the first three of four laps, with 93 riders tackling the 72.8km race.
The race finally exploded with six kilometres to go, with Bertizzolo launched a searing attack on the steep Alto de Compostilla climb, with only Diderikson able to follow.
But with two kilometres to go the peloton had regrouped, and the Dane managed to latch on to a strong leadout from Italian Katia Ragusa to defend the Rainbow Jersey she won in Florence last year by half a bike length.
"Im just so happy, it was a really hard race," said Diderikson.
"The Italians were attacking and I was just trying to hold their wheel. I must thank my team mate Pernille (Mathiesen), who did a great job keeping the race together."
Junior Women: 72.8km, Ponferrada,
1 Amalie Dideriksen (DEN) 2hr 02min 59sec
2 Sofia Bertizzolo (ITA)
3 Agnieszka Skalniak (POL)
4 Nikola Noskova (CZE)
5 Lisa Klein (GER)
6 Great Richioud (FRA)
7 Jeanne Korevaar (NED)
8 Jelena Eric (SRB)
9 Saartje Vandenbroucke (BEL)
10 Mathilde Cartal (FRA)
The first five days of the UCI Road World Championships will be live-streamed on the Cycling Central website, while the final two days will be broadcast on SBS ONE and streamed online.