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It follows his win in the European Youth Championships last summer.
Heyward, 18, led from the front and never fell behind as Norway's pre-race favourite Jakob Ingebrigtsen fell on the final lap.
The Cardiff AAC runner broke a 33-year-old Welsh junior record to qualify for the Championships.
"It feels surreal to be honest. Coming in ranked 10th on paper just shows that times don't matter," Heyward told BBC Wales Sport.
"I knew [pre-race favourite] Jakob Ingebrigtsen was strong and everyone was raving about him. But I knew if I used my brain I could beat him.
"I don't go into any race not trying to win. The whole year, the times I've got up early to do gym sessions with the boys, all that hard work really does pay off and I'm just so happy."
"It's amazing [after winning last year's European Youth Championships too] but at the end of the day it's only Europe.
"Next year it's the World Junior Championships so hopefully I can look forward to that and see if I can pick up some medals in that one."
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Wales' Jake Heyward won Great Britain's first gold of the European Junior Athletics Championships with a commanding victory in the men's 1500m.