Paralympic, Commonwealth and world champion Hynd, 19, won in four minutes 31.66 seconds, with team-mate Josef Craig taking bronze in the same race.
There were further golds for Hannah Russell (100m backstroke S12) and Steph Millward (400m freestyle S9).
Amy Marren finished second behind Millward to take silver.
“I am absolutely amazed,” said Russell, who was competing on her 18th birthday. “I never realised with my class and being visually impaired, I had no idea where I was in the race. I tried to keep it together and do what I’ve been doing in training. Obviously it’s paid off, it’s fantastic.
“I’ve come away as a European Champion, it’s a dream come true.”
Elsewhere, Ukrainian world champion Dmytro Vynohradets completed his sprint double in the men’s 50m freestyle S3 and Norway’s Sarah Louise Rung (1:44.64) retained her 100m breaststroke SB4 European title. But the seven-time world champion said she was “hoping to go a second faster”.
Five medals in the Tuesday morning session at the Pieter van den Hoogenband Swimming Stadium added to the eight from day one of the championships and put GB in second place in the medals table behind Ukraine – ahead of the afternoon’s action.
Multiple Paralympic medallist Matthew Walker goes in the 50m freestyle S7 final and Susie Rodgers competes in the female race in the same category – looking to add to her 400m freestyle gold on day one.
Sascha Kindred gets his sixth European Championships campaign under way in the 50m butterfly S6 final and in the 100m butterfly S10, GB’s James Hollis and 17-year-old Kahoru Harazawa go head-to-head.
Tuesday’s action is rounded off with Eleni Papadopoulos and Gemma Almond competing in the 100m butterfly S10 final.
The British team features 30 para-swimmers competing in multiple events over the six days.
The championships run until Sunday, 10 August.