The Italian, who featured in a gold medal winning 4x200m relay team earlier in the week, eased home nearly a second and a half clear of Germany's SIlke Lippok.
She said: "Everyone was expecting me to win but it wasn't so obvious for me.
"Lippok can swim a lot better than she did today. I had to be very careful."
On Sunday, the 23-year-old competes in the 400m freestyle, at which she is world champion.
Mireia Belmonte García (ESP) - 16:05.34
Éva Risztov (HUN) - 16:10.04 NR
Erika Villaécija García (ESP) - 16:15.85
Milorad Čavić (SRB) - 51.45 CR
László Cseh (HUN) - 51.77
Matteo Rivolta (ITA) - 52.40
Federica Pellegrini (ITA) - 1:56.76
Silke Lippok (GER) - 1:58.19
Ophelie Cyriell Etienne (FRA) - 1:58.23
Mercedes Peris (ESP) - 28.25
Arianna Barbieri (ITA) - 28.31
Sanja Jovanović (CRO) - 28.31
Radoslaw Kawecki (POL) - 1:55.28 CR
Péter Bernek (HUN) 1:55.88
Yakov-Yan Toumarkin (ISR) 1:57.35
Damir Dugonjič (SLO) - 27.32
Fabio Scozzoli (ITA) - 27.49
Panagiotis Samilidis (GRE) - 27.64
Germany - 7:09.17
Italy - 7:13.10
Hungary - 7:13.60