Hosszu won by a comfortable 4.37 seconds from Britain's Hannah Miley, with Hungary's Zsuzsanna Jakabos in third.
Minutes later, Hosszu raced in the second semi-final of the 200m backstroke and qualified for the final with the third fastest time.
She is also down to race in the 200m medley, where she holds a second world title, and in the 100m freestyle, 100m backstroke and 100 butterfly at the week-long European gathering in Stratford venue of the 2012 London Olympics.
"Honestly I was a bit surprised at the start, I felt like I just took up my hold and if you look back I was really behind, but I kept my cool and I was able to go a bit faster than in the morning. I'm really excited with these times," she said.
Asked if she might attempt all six events at the Olympics in less than 12 weeks' time, Hosszu replied: "I'm still open to what I'm going to swim in Rio. I haven't decided yet."
In the men's 400m freestyle final, Italy's Gabriele Detti took the lead just before the halfway mark and stretched out his advantage in the closing stages to win comfortably ahead of Henrik Christiansen of Norway and Peter Bernek of Hungary.
There was disappointment and some mystification for home fans when Briton James Guy, the world silver medallist in the event, trailed in 9th in his heat.
Guy, unshaven and wearing a loose costume apparently to test himself in adverse race conditions, came 27th overall in the heats in a time of 3:52:91, 4.48 seconds behind the slowest qualifiers for the final.
But with the top competitors firmly focussed on peaking at the right time for Rio, and many maintaining a heavy training schedule around this week's championships, it was hard to read too much into his performance.
In Monday's other finals, the Netherlands beat Italy by 3.88 seconds in the women's 4x100m freestyle relay, with Sweden in third. In the corresponding men's event, Olympic and world champions France took the gold ahead of Italy and Belgium.
(Reporting by Mark Trevelyan; Editing by Ken Ferris)
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